front engine to rear engine??

Anything's possible but the more complex / involved the modifications then probably the more expensive it's going to be to conform. Rules say any modifications to chassis or monocoque means BIVA. I suppose you could get away with relieving a bulkhead to clear cylinder head / exhaust, enlarging gearbox cover for fitment of an alternative unit without a test but doubt it with fitment of an engine at the other end of the vehicle.
Do you already have a vehicle you want to do this to in mind ? If not then the good old VW chassis with the bodyshell of whatever shape you want is a good start for dreaming.

Paul H

Basically there's a 99.9% likelihood that to keep it legal you'd need it IVA testing. If you modify any structural part of a monocoque chassis then it looses the right to its original identity, and will need IVA testing to get a new VIN number so it can be road registered again.

If the base car's type approved (most cars early '90s onward) then it should be fairly straight forward to get through the test as long as the work done to it is to a good standard. The real downside being the cost of the test as just under £500 for the initial test, and if I remember right ~£100 per retest.