In European and North American club racing almost anything goes.  For a British owner at FIA events in Europe you can only use a round tube Ginetta G4 with a 1000cc engine and an A series axle (this should not be so but it is the current thinking).  This will give you GTS papers for which the Ginetta G4 was homologated.  You can run an original car or a continuation car with a historic technical passport.

Otherwise you can run GTP papers providing your actual car is running to the specification that a G4 ran internationally in period (prior to 1966) and you have proof!  A G4R (Independent rear suspension and Lotus twin cam engine) raced at Sebring so it should be possible to get an HTP for one. European owners seem to get GTP papers a little easier.

The Classic Sports Car Club allow any G4 to 1960s specifications i.e. Both round and square tube cars of any age are eligible as long as they have period engines.

HSCC will only allow 1960s production specification or FIA cars.  So unless you have an FIA car or a production 1000cc, 1200cc, 1300cc or 1500cc car you will struggle as twin cam engines and independent rear suspension are not deemed "production" and are effectively banned.  Spoil sports.  Too many scared Elan drivers there we fear!   However, Dave Randal has achieved outright wins and the overall championship with 1500cc.