The 250 engine is a 2.3 Volvo Penta block connected to a SP-C drive.
Both the 4.3 and the 5.0 engines are based on GM Chevy blocks. These engines come with a SX drive (usually).
So with the two different manufacturers making the basic block, it is not easily possible to mix a new type engine (4.3 and 5.0) with the older drive or conversely is it not very easy to put a new SX drive on the old 250 engine.
It is possible, but the amount of money needed to allow the new type of engines to work with the older SP-C type drive makes it not worth it (approximately $3,500 for the adapter parts alone).
So it is better to either replace the whole package with a new package or purchase a rebuilt 250 engine somewhere and reuse the SP-C drive.
Volvo Penta offers a V6-200 engine/SX drive package for $14,500 (engine, transom adapter, transom shield, drive, basic panel and prop).
All prices are quoted in U.S. dollars. While Marine Parts Express will attempt to minimize the amount owed, any duties or tariffs are the responsibility of the purchaser.
Thank you and let me know if I can help further.