I also have a 77 240d with 202,000 miles.

It uses a little oil between changes and smokes a little so I have thought of all the same alterations.

After looking at the cost of labor and parts etc. I realized that it would be much cheaper to just look for another car with a larger motor or standard transmissions. They are cheap compared to the costs involved in conversions. Could probably buy two replacement cars for the costs of fixing mine and I probably would be able to get a newer one in the process.