If there are time/programme restraints, you could consider running 40-49, 50-59, 60 plus, all-in (ie, regardless of capacity). That way, the rider could enter the bike he felt was most suitable for the circuit/conditions and get stuck in. If there is plenty of time and the programme can run freely, leave it alone, and run it as it has been satisfactorily run.
But historic roadracing is now choking with too many eras/capacities, and struggling to cater for all their classes at nationals level. As a result, any crash or breakdown at their nationals compromises an already-tight schedule, and races are shortened or dropped, that's no good.