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.

It doesnt look too bad. How does the car perform now with the modification ?

Did you check the thermostat and also that the spring loaded radiator cap was ok ?