RE: 250cc Briggs & Stratton for large scale planes?
With the right equipment and some ingenuity it could be mounted to an air frame but even if one was satisfied flying nice smooth level circles, never banking more than about 30 degrees, never going inverted, never going vertical, I seriously doubt there is an ARF anywhere that could take the pounding a 250cc single cylinder would punish it with for more than a few minutes before pieces starting falling off.
There are many more reasons it would not work well but once parts of the airframe start falling off they're all moot.