In practice, Tim, we've seen more of this sort of thing in cars that aren't able to be 'made in the likeness of.'
Cars of the fifties aren't allowed to be built up, but a number of genuine specials are running either in a form they got rebuilt into a bit later in their racing history, or they were modified at a time when Historic Racing was new here and have been allowed to get away with it.
One of these is Lou Molina's
Vulgarilla, an MG Special that was in fact the
Platypus MG in the fifties with an Austin Healey engine and monoposto body. It was Lou's concern for the genitalia over top of the driveshaft that had it changed, apparently!
The photographed car sports a grille modified from an International truck, inverted, by the way, and has a Chrysler gearbox.
This is more typical of the creations that come of the allowance to build these cars. An old British Chassis is a good start, but a lot come on American chassis, with an American engine generally, because they are large and loud, and a body of some kind or other.
This is more or less like this car was, and so typical of many...
This was built in the immediate post war years with a Ballot touring chassis from the twenties, along with its running gear, and an Oldsmobile engine and gearbox from the mid thirties. The body style was meant to emulate Peter Whitehead's ERA, which it did nicely.
One car recently finished has a 5.7 litre aero engine from the twenties in a Buick chassis of the same era, while one of the really quick ones is considered to be having a bit of a lend of the rules for Group J (pre-1931), an Amilcar chassis with an AC six that goes very well, thank you.
I would like to see it happen for cars through to the mid fifties or so, but some say this would just lead to an abundance of Jag-engined things, of which we have a couple of real ones anyway.
Yes, I can see how it might be a drain on some of the PVT cars in your land, but here there is too many rubbishy old things about that can be more readily obtained that it's not such an issue.