Anything is doable. The more modern diesels have more complex wiring though. Stick with old school diesels that need 3 wires and a fuel supply 1.9cdti has been done in a Corsa B, which I think would be quite nice in a mk3 actually. But not a 2 minute job by any means.
I want a simple old school diesel turbo but bigger than 1.7 so that could be a x20dth? I intend to get a donor car then swap the lot over to make things easy. I'm not so interested in the y20's and their complex wiring. I wonder if there is a 2.0 Isuzu variant as I hear the Isuzu lumps are very moddable. More bhp is the end gain here but can't afford petrol running costs.
The main reason is for more power. More speed. My van is rarely loaded with any weight but feels very sluggish. I'm used to my c20xe, I'd be happy with 100bhp I think but would also love an engine I can mod.
You could spend that much time messing around trying to get a newer larger diesel engine in that it wouldn't offset the cost of just having a petrol engine. The best and easiest option for you is as stated, put the 1.7 isuzu engine in and fanny about with the fuel pump etc. Everyone will rave and tell you how great they are when they're tweaked, but be aware this will make them less reliable in the long run. People see the isuzu through rose tinted glasses and remember them as much faster than what they were. Don't get me wrong they were good back in the day and a lot better than vauxhall's 1.7 effort, but the Peugeot 306 1.9 hdi in the same tune would muller them every time .