Repairing what you have will likely be less expensive than swapping in a different engine, or brakes, or suspension, etc. There are lots of unknowns when you do that. See if you can find a local car buff that can sit down with you and come up with a list of work, in priority order. Some of the things will fall down the list, others will need attention now…
One advantage to fixing what you have is there’s a pretty enthusiastic group of Metropolitan owners that can help you. Put in something unique and you’re on your own.
Have you joined the MOCNA?
http://www.mocna.us/