The Ministry of Transport test (MoT) only tests the vehicle’s roadworthy standard, brakes, lights, tyres, etc.
It does not matter if they are not from the same manufacturer era or planet!
Just so long as it hangs together on the day.
There is a fair amount of discretion available to the tester (as he can be prosecuted for passing dangerous vehicles). Regulation is generally down to insurance companies asking “Are there any modifications to the vehicle?”.
There are a lot of Chops, Bobbers and such trundling around that on paper are Hardly Driveable 125 or 250s! Usually because they have the frame number of the old AMF 2 stroke roadster/trail bikes.
Another favourite is the old Series 2 Land Rovers with full time 4x4, coil springs and a V8. Mostly as a result of a workshop collision between an old Range Rover and the remains of an SII (Mostly just the VIN plate). Although there has been a bit of tightening of the regs over that now.