convert series 3 2.1/4 petrol to diesel

The engine should drop in OK, use the clutch to match the gearbox.
As well as the obvious differences the diesel has different starter, and manifolds, I can't remember if the 88 & 109 systems vary. The fuel pump is different. Beyond the engine all the pipework is different, again if the 109 had a rear tank it will differ from the 88. The tank it self is different, as the diesel has an extra hole for the fuel return, however for many years most suppliers have only supplied the Diesel tank with a blanking plate for use with petrols. The fuel gauge sender unit is different, but this is only because the diesel has an extra terminal to work a Low Fuel Warning light.
Obviously you will have to sort out wiring for the heater plugs, but if you put in an extra switch, you may be able to use the petrol ignition switch. You will need an engine stop cable. If you have a Servo you will need a butterfly flap on the inlet manifold to create enough vacuum (or find a vac pump somewhere) If the doner vehicle doesn't have one they are like rocking horse ****
All in all nothing too difficult, but maybe more than initially meets the eye.

I'm in Luton, I've got a 2 1/4 Diesel engine, needs a rebuild, but free to anyone who wants it, plenty of spare parts for it too. I swapped it out for a 2.5N/A in mine.

I have seen a petrol converted to diesel some years ago, he changed the head, pistons, and fitted the pump, it ran but not well, it turnd out the cam shaft was wrong and had to be changed, then it ran fine, later it snaped the crank shaft, the petrol is cast iron the diesel is cast steel,
buy an eary sherper lump and fit it straight in