Re: max size motor possible for in frame build
The fact that too many people seem to loose focus of is that we're dealing with a bicycle. It has a bicycle's frame, wheels, brakes and was never designed to be engine driven. Keeping the power to a reasonable level, under 3hp is a good number to start with, would be prudent for anyone new to the hobby and not familiar with riding a bike or a motorcycle, for that matter.
I agree with Al; when the first question I see from a new builder is "How can I get more power/make it go faster?" I cringe and back away from offering advice other than this:
Build your first bike for the mechanical experience and concentrate on reliability. As the engine accrues more milage and is properly broken in it will naturally produce more power and probably and eventually all the power and speed you'll need for a bicycle.
That's just my common sense approach and advice to a new builder.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"