These are some if my favorites. Let me know if you have some others to add!
https://www.facebook.com/groups/moto.kinetics/
Suspension/chassis/kinetics
https://www.facebook.com/groups/671081149638729/
Two-stroke research and development, advanced
https://www.facebook.com/groups/117664586824/
RG500 and RGV500
https://www.facebook.com/groups/119904604696500/
AHRMA racing
https://www.facebook.com/groups/636379916490638/
Australian Smokers
https://www.facebook.com/groups/238340878030/
Keep Two Strokes Alive
https://www.facebook.com/groups/616874758355349/
North American RZ350 Owners Group
https://www.facebook.com/groups/1957917217753994/
Vintage Suzuki Owners
https://www.facebook.com/groups/375063336023003/
Bespoke Sports Motorcycles 70s-80s-90s
https://www.facebook.com/groups/vint.japan.2stroke.club/
Vintage Japanese 2-strokes
https://www.facebook.com/groups/750586571645154/
The TZ Group
https://www.facebook.com/groups/team12himitu/
Yoshimura Racing Supporters
https://www.facebook.com/groups/twostrokepartsbuyandsell/
Two-Stroke parts buy and sell
https://www.facebook.com/groups/526441327454399/
2-Stroke Sportsbikes
https://www.facebook.com/groups/Suzukiracing2strokes/
Suzuki Racing 2-strokes
https://www.facebook.com/groups/423011861435291/
Classic Kawasaki
https://www.facebook.com/groups/171330347015906/
2-Takt Germany
https://www.facebook.com/groups/Hondaracing2strokes/
Honda Racing 2-strokes
Good Facebook Groups