my HT that im using currently has over a year of abuse, over 50km rides daily, mostly full throttle. its been through four frames... had to replace one crank seal so far. (48cc)
on the other hand, i had one HT that couldnt get me the 5km to work, ever. (a 66 or "80")
miles? 30 and 3 approx.
get a cromo frame, mountainbike with straight seat stays. v brakes, or cantilever at a minimum, front and back. having just fitted a "dual lever"... i regret not having done so earlier! both brakes on one lever is so much nicer!
avoid fat tubes, theyre hard to mount. dont drill holes in frames.
accumulate spare parts. stick to one size and supplier....when you find one that doesnt die instantly
cant say who, im not in the US. ignore 99% of the advertising hype. its people trying to make money, not help. IMHO, bigger is NOT better.
friction drive is the pits when there is any moisture on the roads, and as for diesel...oh wow.
and so far, having never seen a fourstroke kit that doesnt require offset cranks... i avoid all fourstroke kits.
ive also had(rackmounted) 3 HS 50cc fourstrokes die. one left me stranded, the others left someone else stranded. i wouldnt trust them for constant use. honda...well, honda is another story
i want one for my kayak! cus i dont want to be stuck up some creek 50km (or miles!) from help with no engine... (and if i leave the river and head down the coast to the next river...oh wow. i need reliability!)