#1 stay away from the crap kits!
This is where you can get the good stuff
http://www.bicycle-engines.com/4-Stroke-Engine-Kits/
Get a Real 4-G transfer case which is what it is all about anyway as the engines are all the same.
Just a tip.
Not really a reply to u sir, Just sayin, the bike snap illustrates 2 fundamentals for me:
The motor is a tight squeeze in that frame. I would opt for less style & MORE SPACE.
NB, u have to stop too. Caliper I could live with on the back, but on front, w/ extra speed & weight of a MB, disks with the larger disk rotor (20%+ better stopping) on front seems very wise. NB also, caliper rim brakes wear/weaken the rims in time of course. Sounds a hassle.
Front Disks are a function of the front forks. They must have mountings etc. integrated in them. I gather its hard to find in this style bike.
I can see this as a comfortable ride due to mass, but a recycled steel or alloy MTB would work fine and save ~10kg.
You dont have any gears , so motor torque may trump power re hills.
The good news is, u will be surprised at the money u save if u pass on a car. Folks dont realise the constant costs til they stop.
Know the real enemy. Above 15mph, by far, the greatest resistance is from wind.
If a pull start adds cost/hassle/weight, it seems mostly optional to me. Just pedal>jump start it.
4 stroke does mean u simply fill at the pump. Mixing oil sounds a hassle.
4 stroke may have more torque? Confirm anybody?
Those chain drives are a vicious accident waiting to happen. Think about a guard. Think about a good joining link in the chain. maybe advantageous to easily disconect it so can pedal like a normal bike better in a jam.
Think about a bit of electric wiring. One decent battery for all the lights sounds less hassle & cheaper than multiple packs, or at least standardise on one rechargeable cell type, like 18650, so e.g. 2 spare charged cells suffices for 4 lights, & one charger & procedure. Maybe contrive to just charge in situ where u park the bike - simple.
An electric jacket & gloves sure sounds good to me if its cold, but i gotta get to work. Again, a single battery & wiring loom sounds good. Maybe Even a little generator for extra charge is a simple matter now.
u can simplify by ignoring tail lights. thats easy w/ common dry cells & $2 red led flashers. Focus on the handle bar area & Worry about wired tail lights later.
OTH, helmet lights are good too.
Its the batteries for all this good stuff that is the expensive hassle, so my point being, if u agree, it needs thought. to make 1 good battery suit all, u gotta think about volts etc. of devices being compatible, and choosing your accessories correctly.
Whats your plan b if u break down? Will it fit in u buddies car? Just saying, my plan b is the back seat of a cab for my ebike, so a light mtb w/ qik release front rim appeals to me. It means i dont have to stress about a repair kit for a roadside repair thats bound not to work anyway. i will simply worry when and if it happens. its worked for almost a generation now.
Doable on a car bike rack too. Not too heavy.
speaking of plan b, where do you see pedaling coming in? I dont think pedaling this 20km home is a plan b. On a mtb, with the motor chain removed, riding home would be ok in a jam. Pedal assist is a surprisingly handy supplement/backup at times. What it lacks in power, it has in torque. Just a little extra when motor is really grunting (lacking torque) on a hill, makes a disproportionate difference, w/o being sweat inducing.
I have been there for many years (no car, broke, a long , regular and important commute (visit kids)). A piece of crap 25cc brushcutter motor friction drive MTB- awful torque on hills, but damn good wheels really. The only gear of the 24 gears i used was the lowest one - on hills that stumped the motor, but here is the thing, the motor still did most of the work, it was really light pedaling, it just put the motor in a more comfortable rpm range & between us, the hill was no problem.
an important thing for many workers is to never HAVE to get sweaty. as above, pedaling a bit doesnt have to mean sweaty.
You dont mention the distance of your commute as i recall?
Dunno, I am a noob w/ these types of bike too, but i suspect clutch is critical. a crappy clutch will drive you nuts. the engine must disengage fully, easily operated and maintained, and reliable. Maybe hydraulic is good if it exists. Cable arrangements often seem crappy and stiff. I think others are saying the same in other terms/contexts - reduction box etc.
Its the transmission as much as the motor, maybe more so.
The good news mate, is i am confident u will love it.
A big surprise i think, will be that it really is point to point transport, and how pleasant and time saving that is. U dont circle looking for a spot in the lot, park, lock, walk to shop etc. U ride to his stoop, lean it, & stroll 20 feet to the counter. U realise so much of driving is waiting and detours, almost all of which simply disappear, when u are not the size of a parking space any more, more similar to a pedestrian, with most of the options, and much faster when the coast is clear.
personally, i would prefer innofensive accoustics. i want good vibes from folks, not annoyed ones.
if u opt for a 4 stroke, do several qik oil changes, til no metal filings glint in the drain pan in sunlight.