Stu,
I would suggest visiting the previous owner maybe with a heavy object about his head. Or just shake him warmly by the neck coz it looks like it was in a thump and he knew it had taken damage and palmed it of on a poor unsuspecting hoodie.
Seriously though if you get a spare injection engine then you shouldn't have an enormous bill to get the thing running again.
Was there damage to the piston in the cylinder that had the problem? if so you need a piston or a set (alright these could be pricey if they are special for the turbo application).
The block from your injection engine is suitable (it should be a 205 which is capable of coping with fairly mental power and it will easily overbore to 2.1).
The crank will probably be the same and is OK for lots of power if you dowel it to the flywheel (they tend to snap the bolts on high torque motors if not).
The auxiliary shafts should be the same so you have a spare in the injection (or carb) lump. The oil pump will probably work but I would suggest a high capacity (maybe high pressure as well) one to ensure the turbo has enough of the slippery stuff (£60ish from memory).
Now you're looking at con-rods - well the injection ones are supposed to be OK for 7500rpm but not sure about the extra loads the turbo generates but you should have 3 good ones so can you get a single one to match as a replacement for your broken one? Oh and you'll be wanting new heavy duty bolts as well ;>
Is the cam and valve gear OK? a new spray-bar is only a few quid so the head should be relatively easy re-use if they are.
Barring the piston/s and con-rod/s you shouldn't be looking at more than a hundred quid for the injection block to be bored and the work to be done on the crank. Maybe £200 including the oil pump, shells, gaskets and various bolts. Alright it's annoying to have to pay when it wasn't expected but what price having that sort of engine?
Start looking for pistons and rods first as they will be the most difficult/expensive stuff to find so they may end up being the death of the project rather than anything else.
Good luck
Iain