I’ve always looked down my knows at lumps; be it Jag, Ford, Chevy or the like.
if you want more go, then keep the powerplant within, at least an option spec of what the car’s model year.
If the car came with a V12, (and are so inclined) then make it so. V12’s can be acquired pretty cheaply. the caveat is getting one with the proper block coding to match the year of your XJ. Then there’s getting the proper FI module & wiring harness, for that year V12, or the manifold & carbs for an early Series II, there are companies that re-cast these parts, and others that sell rebuilt SU carbs (especially in the UK). The SU’s aren’t particularly expensive the recast manifolds, on the other hand are, and will set you back about $3,500.00.
For me, and my conundrum
I would love to have an series III XJ12, but sadly I don’t.
I have 2 Series III XJ6’s and a spare matching year '85 V12 (50,000 miles on it) with the ECU and wiring harness and transmission in storage. I picked up for $400.00 on craigslist from a parted out XJS. I could lump one of my XJ6’s to an XJ12 but then it would be a lump (this kind of goes against the above paragraph. Not to mention it’s a proper pfaff to retro fit one. It’s essentially a complete running gear replacement. and stripping of the interior for the wiring,
So if you were so inclined, putting the car back to an XJ12 is very doable, and can be done for under $2,000 and gobs and gobs of (your) time.
The $16k is for a expert V12 rebuild. The most of parts for these engines aren’t hugely expensive. The cost goes up because there’s just a bunch of them aka 12 pistons, 12 liners, 24 buckets, 24 springs, 24, valve guides, 24 valves, 12 injectors. I think you get my point. As for the special tooling, most can be made with a bit of ingenuity; some aren’t necessary, but just make the job 1000 times easier. If you watch ebay, you can find the special tools; at a reasonable rate.
With a whole lot of patience, due diligence of monitoring scrappers, Craigslist, this forum sale website, ebay, offerup and the like, You CAN get the bits necessary to put the car back the way it was at a very reasonable rate.
For what ever its worth