I built my motorized bicycle on a vintage 1988 Specialize Rock Hopper mountain bike with a very small 16.5" frame and 26" wheels. This frame is tiny and I was very worried about it fitting. It barely made it, I had to use a low profile air filter (didn't want to use a z-tube to move the carb outside of the frame) and it was a challenge to fit the spark plug boot over the plug at first, other than that the build went fine.
The spark plug clearance soon became a non-issue, cause I tossed the crappy kit boot and wire and put an automotive plug wire and boot on my cdi. The automotive boot is rubber and thus flexible and so it will go over the plug with very little clearance (unlike the rigid plastic POS boot that comes with the kit). There is also an E3 spark plug that works very well on these motors which is MUCH shorter than the kit or NGK plugs (forget exactly which one, post back if you need the info and I will find it for you), if you have trouble with the boot try the E3 plug or go with the automotive wire and boot (I highly recommend you change the wire and boot anyways, they are a weak link in these little motors)
There is a diagram out there that shows the minimum clearance to fit the motor, I recall looking at it when I first built, not sure if you've seen that or not? I still worried about the fit even after confirming it should work out with the diagram, it was a little more challenging to build on such a small frame than a bigger one, but in the end it worked out great.
If you build on the small frame you may have some cabling issues (as I did) because things are so tight you need to make some fairly sharp bends, specially on the throttle cable. I had some problems with cable drag as a result which was screwing with my idle. I ended up using better cables with insert linings which had much less drag, and also used some flexible noodles to make the cables traverse the sharp bends more smoothly which worked out really well.
Good luck on your build....
