Class C and £450 I'm afraid and from what I understand using anything other than the engine that was in the donor car may not even get you a date for IVA unless you are a registered manufacturer. Would be classed as a low volume vehicle then allowing manufacturers to use bike motors etc.....BUT they have to meet 2009 regs on emissions which rules out Rover V8's apparently.

If in doubt just use the motor that came free with the donor car and keep it simple for IVA then change it afterwards....simples!