Andy,
Not sure whether there was a 214 flywheel that fits a 28hp. I replaced a 28 in an MW with an army rebuild 214 straight out of the box, dated about 1990. It was intended for a TK, probably RAF or RN because army did not have TK trucks with 214 fitted. It drops straight in with minor adjustments on throttle linkage, engine breather, top hose...EXCEPT, the flywheel housing was different, so I changed the housings over, also the flywheel because of a difference in diameter, so ended up with a 214 with 28hp flywheel and housing. The Brake horse power of a 28hp is about 72bhp, where as the 214, which is same capacity, is 100bhp. It makes powerful little truck, which will now hang on in top gear much longer, especially good for towing a trailer, which its owner wanted it done for.
A 300 petrol, or 330 diesel is much to big to go in and major surgery would have to be done, although it may be possible to put a 220 4 cylinder diesel in, but again it would need new engine mountings fitted to the chassis.