I guess the proper question is, "If parted out, will I be able to get my money back out of it?" Answer is, probably. IRS is worth something, salvage some other misc parts and you'll come out OK (if you give up on it). Check sites like "jag-lovers" for other possibilities and prices for used parts.
The enemy of the V12 engine is heat-they just couldn't keep 'em cool enough under those long low bonnets, resulting in warped or cracked heads/gaskets/etc. I have the book from the site that Unkl Ian posted. It makes everything look pretty simple...I haven't done the conversion yet, though.
As far as "prince of darkness"....true, but I've found most problems can be solved by simply cleaning up the contacts-time consuming, but much cheaper than having to buy new parts.