I wouldn't bother. By the time you have got your engine, bough all th bits you need to put it in and faffed arond putting it in, and then faff around putting it back as it should be 12 months later you may aswell either:
a) just pay the extra insurance for the year because after parts and down time with the car you probably won't have saved much. Also I don't think the insurance will drop a great deal as the car is still a "sports coupe". Ever tried insuring a 1.6 Capri? not a great deal cheaper than a 2.0, not enough to warrant the smaller motor anyway.
b) buy yourself a cheap runaround for the year. This way you can either abuse it and kill it (if it was cheap enough) or sell it on at the end of the year to recover some of your cash.
I would personally do option a if you already have the car, I certainly wouldn't bother swapping the engine for something smaller or a diesel.
Oh and a Petrol to Diesel swap would have to include to engine wiring loom, fuel lines, tank and probably front suspension as the diesel engine will (probably) weigh alot more than a petrol one.
Good luck with whatever you do and I hope this helps..