Mechanically most parts are the same as an E type or other Jaguar of the same year of production. Where the Mk10 / 420G fall down is body panels. Whilst there are a couple of minor repair panels available there is very little compared to E type or Mk2's. I believe a few S type / 420 parts can be adapted to fit.
Some smaller parts for brakes, suspension are becoming difficult to find as are some bits like chrome screen trims.
Interestingly, interior trim is readily available from Aldrige Trimming in all colours. Full trim kits are available or any part thereof. I plan to refurbish my 420G with parts from Aldrige.