It really seems to be more a matter of preference than economics. Some people want strictly honda, so an aftermarket engine is out of the question. If you want to go with one of the 88cc kits, they are roughly the same price give or take as a lifan 125 or 134. With the lifan, you get all new parts, maybe not all the best, but all new parts just the same. The honda parts will be 20 years old depending on what you use. The xl and sl70 engines have the advantage of the clutch and kick start and also the auto advance distributor, which the atc70 has none of those items. If you want more power, than it seems to me the auto clutch just can't take the power that a manual clutch can. The honda engines are pretty reliable and since they built so many, parts aren't that big of a deal yet. So it really comes down to what the individual wants. If displacement is what you want, than the aftermarket engines are really the only way to go.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90