Engine on 18" frame?

Probably, but to be safe it should at least be 20". I ordered an 18" frame once and had to return it because it was too small. You can still use 18" frame but with a rack mount engine.

I used a 18" Diamondback Outlook. It's tight- should have got the 20". Not impossible tight just annoyingly tight. had to find a happy medium between just enough clearance to get the plug boot on and clearance for the throttle cable on top and air filter on back. Its kind of a pain.

Should work I put a 49cc on a 17.5 mountain bike, but as previously posted it is tight, but possible

try a slant head it should help out with some clearance issues

Okay, so I was seeing that some manufacturers say their motors will fit "24 inch and larger bikes", even though the engines themselves are much smaller.. Is the 8.5" clearance above that they recommend just a precaution issue or is it a legitimate concern?

How Hard do you want to work on your bike.
The carb will not fit right.
The sparkplug will not fit right.
The throttle cable will not fit right.
Maintenance becomes annoyingly hard.
I have tried it.
I had to pound a recess for the plug.
end into the frame.
Drill a hole for the throttle cable.
Use a foam airfilter as it was the only wa to even have an air filter..

Maintenance was a drag.
Pull the carb to adjust the needle or clean the jet.
Pull the engine to clean the sparkplug.

Get a bigger bike.
It looked nice but was way to hard to maintain.
a 24 inch bike with a happytime is possible not enjoyable.