Foe F3: A 400 will probably be the most competitive.. However someone could dust off a 125c gp bike and it would clean up.
Here are the rules:
Machine limits per formula category rules unless otherwise noted.
Unlimited displacement 2 & 4-stroke singles unless otherwise noted.
Up to 125cc 2-stroke Grand Prix machines
Up to 396cc 2-stroke water-cooled twins
Up to 410cc 2-stroke air-cooled twins & triples
Up to 560cc 4-stroke water-cooled multis
Up to 570cc 4-stroke air-cooled 4 valve per cylinder multis
Up to 600cc 4-stroke air-cooled 2 valve per cylinder multis
Up to 700cc 4-stroke water-cooled twins
Up to 900cc 4-stroke air-cooled twins
Up to 1200cc 4-stroke air-cooled pushrod twins
Among the eligible machines, EX250, EX300, RZ350, RD400, EX500, GPz550 and
Hawk 650.
Air cooled two valve per cylinder Ducati 900, Yamaha FZR400, Honda CB1, Triumph
Thruxton 900, and XR1200 are allowed.
250cc 2-stroke GP replica machines Aprilia Cup RS250, TZR, NSR, RGV, etc. are allowed
but limited to Superstock mods.
No SV650, no Ninja 650R, no Ducati Supermono, no Aprilia 4 stroke V-Twin, no 250cc GP
machines allowed.