Perkins what, 2.0l 'Prima' or 4204 Dodge walk-thru motor, or any of the more quixotic Perkies as used in Jap motors, catapillar bull-dozers or even Hydrovane builder's compressors?
For the TDi conversion, use the search facility, or just click through a couple of pages of this board; question seems to get asked almost daily!
Its an oft done conversion, as the motors are readily available from dead Disco's, and it fits to the series bell-housing like it was meant to be bolted there, as the TDI shares the same flange drillings as all the series landies back to day one, though I think that they changed stud sixes to metric at some point and it fits better if you ream the bell-housing holes out.
BUT, in my opinion, its not a 'good' match, as the way a TDi makes its power puts an awful lot more strain than the numbers suggest on the series drive-line, which even if 'good' is being asked to deal with well over twice the power it was ever deemed good for.
Its to do with the way the power is applied, rather than the actual amounts, TDI power comes in with a big 'bang' from tick-over to 2000rpm, hitting the gear-box like a sledge hammer, and normally doing damage, or at teh very least hugely accelerating wear.
De-Turbo's TDi unit works better and gives the drive line an easier time.
But of the two, I think tha the little Perkins 2.0TD is quite a cracking engine, and though an out-dated conversion, still got a lot to commend it. Not as straight forward as a TDi, but knocks out useful power, (90 - 100bhp, almost as much as a 'good' TDi) without it loading the transmission, and reputedly the most ecconomical engine ever fitted to a series, with reports of 40+mpg not being uncommon, where the TDI rarely betters 30.
4203 Perkins, in normally aspirated form, is an old dog, loads of grunt, that twists series drive line bits to bits, and but very little 'go' as they dont rev out.
Turbo'd version works better, but not much and puts even more strain on teh transmission, and neither are wonderfully ecconomical.
But use your mouse, its ALL been said before......