If you want horsepower and the reliability of a Briggs & Stratton, the model 15 is a great choice. It may require slightly more work, but the end result is worth it.
If you try to bore an Animal block big enough to put a large diameter sleeve in it you will bore all the way through the old cylinder into thin air, or it will be so thing you will blow the cylinder off.
One of the GREAT THINGS about a model 15 block is that it can be bored and sleeved to accept a 3" or larger piston and maintain its strength.
Most of the Animal / 206 parts fit themodel15. The billet side cover, crank, cam, lifters, push rods, flywheel and blower housing all fit the model 15 perfectly.
It has the same footprint and bolt pattern as a 206, an Animal or a World Formula and fits all the 206 and Animal motor mounts. However, the Animal, 206 and WF heads do not seal very well because of the much larger bore of 3" or more. My billet head works great on the model 15.
If you choose to us a stroker crank you can end up with a displacement of over 300 ccs.
If you don't want to install a cylinder sleeve you can have the cylinder Nickasil plated.
The flywheel side crank bearing needs to have a special bushing installed because the original bearing surface is simply made of aluminum. The part number for the DU bushing and seal kit is 555140.