You would be surprised at the power increase and how robust the small motors are, there is a gokart racing and a drag racing class that uses these side valve motors and they happily rev to 6000 rpm. Check out the link http://www.aviator.cc/briggs.txt
I followed most of this guide and got great results.
I did the following
1.ported block and flowed the valves
2.flowed the carb
3.milled the head and block (a lot)
4.straight through exhaust
5.synthetic oil
6.advanced timing
7.high flow air cleaner
8.one shot nitrous
and another link
http://www.hotrod.com/newsletter/hrdp_0308_racing_engine_jr_drag_racing_league/