http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=26454&item=2424130365
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=26455&item=2424140137
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=26455&item=2424296870
Briggs does not seem to make thier tech data readily avail on the internet but if you ask enough people you should find out soon enough. Better yet call some repair shops and ask them.
You can get a tach for those motors, its called "tiny tach", I think its like $50 though. You might be ok just to assume 4000 rpm for the briggs and 6000 rpm for the 2 stroke. If you do for your 3.5 hp it would be 3.5*3800/2500 = 5 hp lower end from the earlier calcs if they are valid. The way I see it is that you may not be able to buy a lower end with hp that exactly meets your calcs. Maybe anything from 3-10 hp lower end would work good enough.
When I posted photos I would just past in a link to my webpage.
