I’ve known quite a few people of the 1950’s and 1960’s era that did engine transplants and none of these transplants provided reliable transportation. Two transplants that I recall were a 1954 Cadillac engine in a 1951 Mercury. The electrical system had not been converted to 12 volts and my friend push started the Mercury most of the time. Another conversion was a Buick nail head V-8 in a 1953 Studebaker Starlight. The engine made the car too nose heavy.
I think the engine in your Nash Metropolitan is an Austin engine which was also used in the MG. To a car collector, the car is worth much more with its original engine. Furthermore, if this engine is in good tune, it gets good mileage even for these times.