It's not a silly question, and I hope this isn't a silly answer.... but rather than spending a lot of money on a 12-year-old van, wouldn't you get better value for money from replacing the van with one that suits your needs?

There's more to just swapping out the engine/box for a later/larger capacity one. There's the possibility of the mountings being in different position for starters, the wiring harnesses being different, the fuel and exhaust pipework, the immobiliser circuitry etc etc.

It sounds to me that you've committed yourself to this van after all the money that's been spent on it and feel you (or the good lady wife) have to keep it to justify that expense. Unless you plan to carry out the transplant yourself, I should imagine you'll find it difficult to find a garage to undertake the work and if you do, the costs could soon spiral if it turns out not as straightforward as they may think.

Then there's the insurance to take into consideration. You will have to declare any modifications, especially if a non standard engine is installed, and I should think the insurance Co will ask for an engineers report to be carried out before they even consider covering you which is even more expense.

To me, the two courses of action are : spend more money on it to correct the current faults or, call it a day and sell it on/part ex it for something more reliable and suited to your needs.