Antique Small Engine Collectors Club (ASECC.com) – Classifieds, discussion, and info on small air-cooled engines.
SmokeStak – “Blow Smoke – Shoot Flames – Exhaust Gas”. Great discussions but mainly flywheel engines. Some discussion of older Wisconsin & Briggs from time to time.
Enginads Small Engine – Same people as above, but aimed at small engines. Good place to look for or sell stuff.