Peder,
One of our 420G's has a large Cadillac engine (the 502c.i. -- but not a Northstar) with Turbo 400 4-spd auto trans.  Some modifications had to be made to the engine compartment to fit in the engine. 
I think there are more modern engines that could be fitted, with fuel injection and 5-6 speed trans -- and with better mileage and horsepower, all in a smaller lighter unit.

However, after several decades of altering cars, I have come to the conclusion that in the long run (say, when a car is 40-50 years old), the car is worth the most when it is original.  Otherwise, you end up chasing updates.  An "older hot rod conversion" loses its appeal if the engine is an out-of-date transplant.

I know there are several forum members with V-8s -- and there is one with a Jag V-12 in the Mk X.

Here's the 502c.i. in the 420G.  Also pic of the car, with some efforts to keep original (although it has XJS steering rack and brakes, as well as wider wheels). ACTUALLY, willhave to post car pic in next posting, as I can't figure out how have more than one attachment.

For sure, in the US, engine parts and mechanic work (when not doing it ourselves) is much much cheaper -- maybe even by a factor of 10.

Love the car and engine Baloo , but thats a strange number plate , where did you get that ? .

Alan .

Plate came with the car. 
Is there a code to decipher what region that plate was from in UK?  I'm assuming it is British, right?

Car now has antique plates "KAT-BXX"  [all our cars end with BXX, e.g., ZOOM-BXX (911), ZEE-BOX (Z-4), CAT-BXX (the other 420G), DOG-BXX (minivan that the dogs ride in), GO-BXX (motorhome), BENZ-BXX (since retired), BZZ-BXX (retired Scion xB).  However, the 60 Lincoln has "OIL-RIG" antique plates.

A bent plate it may well be , but its still wrong for the car .

A 1967 car should have an E or F for the last letter .

Alan .