check out this small engine im building!

[super vw]

Research the weight of the Thundertiger (or any engine of the same displacment) flywheel. That may give you a direction on how much weight and diamater you may need... only thing is that your using all steel (heavy) internal componets, and i think that thunder tiger (as with most engines) use mostly aluminum internal componets (lightweight) so that may also change things with your selection.

I use to do a lot of 1/8 scale nitro on and off road racing when i was growing up. and the flywheel weight changes a lot of things... but i think for your purpose its not super critical as your not going to be using the motor for any real locomotion of an object?