GEBE set-up for what it is worth
I note some prefer the 35cc Subaru 4-cycle engines versus the 25 4-cycles or the 2-cycle Tanaka engines or other popular 2-cycles.
Last year I installed a nifty 4-cycle Honda GX25 on a single speed cruiser, using the GEBE kit. What I have learned is below.
If you do not plan on running up hills, the wee 25 is just fine and will move you at 20+ MPH easily, and this is fast enough for me. This Honda and the Subaru version are likely the most reliable engines available.
They need breaking in, as I have learned and have been advised by GEBE's Jake. He indicates it takes about 2000 hours (he may have written 1500 hours, will check, but either way, it is a long time) to break the engine in, and I am sure he is correct. He indicates its torque or pull power will increase dramatically. I have noted the engine really has begun to grab after only 10 hours or so.
Now, I live on a paved hill that is, essentially, about .75 mile in length, but it is a climb that required a shift to next to highest ratio gear on my Bianchi when I was able to take the effort (knees). Compared to freeway rise at viaduct - overpass - increase incline five feet or so over length, to give idea of incline.
The reason I installed the engine was to make the climb or make it easier - I pedal-ride with the belt off most of the time.
What happens on the hill is that the clutch begins to slip after a bit and I have to pedal assist with a zig-zag path. I know about the GEBE gear ratios available. (It is getting easier as the engine breaks in, but...) I prefer not having the problem. If the bike had multiple speeds, I am sure it would be better, but single speed cruiser is what it is and cannot be changed due to frame construction.
What I think:
Clutch size is important; engine weight is important; size is important. So, the 35cc 4-cycle versions, Robin or Honda are 8 pounds, versus 6.1 to 7 for the 2-cycle Tanakas, except the 8# monster PF4000.
The PF3300 Tanaka sports a 78mm clutch (large) like 35cc 4-cycles but only weighs 6.1 lbs dry. It is also 1.6 hp, like the heavier 35cc 4-cycles. And, this is a guess, but I think the larger clutch and .5 hp increase will be enough to gain what I need.
Frankly, I like the little 25GX Honda. No oil in gas, light, quiet and small, and if it would take my bum up the hill, without the pedaling etc, I would not hesitate to keep it.
Which all means I am going to move to a large clutch, 1.6 hp Tanaka 33 and sell the Honda. And, if you do not plan to work steep hills and think 20 mph is fast and safe enough, the smaller 4-cycle GEBE kits are the ticket. (You can find separate Honda engines and buy the small clutch kit or go with the whole GEBE package.
Hope this is of some help.