1974 Austin Mini with conversion - reported 1991 Rover 1.3 engine - LHD

Hi to all. Please forgive my newb status to this forum and to this hobby. I've wanted a classic mini since I was a kid and finally found one locally. The only thing is, little is known about it.

I drove it home about 200 miles and it lost power on the way as well as lost use of the tach. I limped home on what seemed to be limited power. The car will periodically (usually in a higher gear) backfire, lose power, and then upon a change in gear returns to its previous diminished power but remains very functional.

It's at a mechanic now. He is very knowledgeable with small engines but I feel he may be a bit in the dark because of the engine. He has little information on it but says one cylinder is not firing. An engine coil was recently replaced and he is assuming it was done correctly and is still good so he believes it is a control module in the engine bay which would also explain loss of tach function.

Tach function has been a problem for a while according to previous owner and the fuel gauge does not work either but he believes this are different problems since the fuel gauge is an old problem. The oil temp gauge works.

Can anyone help me help him work this problem out? I know I need to identify things and obtain numbers to better identify this engine. I have some images if that will help.

Thanks so much to anyone who can provide any help! I've been shopping on this forum for a long time and dreaming about one day being an owner and am excited to have the car.