Not sure about Midget owners but the type 9 conversion is still popular with Morris Minor owners, Some classic Ford owners have gone onto using Mazda RX8 5 and 6 speed gearboxes was they are still plentiful and relatively cheap.

It would be interesting to know what the parts cost on their own and how much of a job it would be.

You would need to work out the offset of the wheel. The 15/185 tyres should fit under the arches ok but it all depends on the wheel offset.

"old train locomotive " - id say first check for leaks around the exhaust manifold

A click on the DVLA's site shows that car was last taxed in April 1994. It might be existing in a barn or a lock up garage somewhere with luck.

Have you tried the Rover 75 and ZT forums. There are a number of issues that can impact the fuel systems, a few more with the 2 litre diesels than the 2 litre petrols, not sure which you have. You will probably get a faster answer on the relevant forum than here though.

It depends what you mean by Modern MG ? For instance I very much doubt MG ZT wheels would fit

How bad is the misfire and can you tell whether it is always misfiring on the same cylinder ?

Also check you are getting a good earth

It would be best to record the noise and post it so it can be listened to.