FORCE races were meant to be for the early (non-wing or at least very simple wings) 3 liter cars, unfortunately the entry levels are extremely variable.
The Cooper Alfa is a regular competitor (lovely looking car, with an Alfa Montreal engine in the back), and the occasional early (e.g. orange) McLaren plus Sid Hoole's Cooper BRM is about it.
The FORCE found there were more poeple looking to run F5000s than early 3 liter F1s.
But there is hope, an Eagle Weslake has been restored, a Cooper Maserati is being restored, and there are various other cars around - some Lotus 49s, BRM V-12s (even the lighweight H-16 made an appearance several years ago), a Repco Brabham surfaced years ago, the Matras have come out of hiding and there is even a running Ferrari 312 out there.
Once there are enough cars running reliably it shouldn't be long before a decent race series follows, which will then draw out other cars.
p.s. Ean Pugh has the Lobster Claw Brabham & a BRM V-12, both of which he has run in TGP races rather than FORCE - on the gounds that it basically costs the same to do both events.