Re: 4stoke or 2stroke
why not just get it welded up ! yes it can be done and will be fine.
get the block cleaned up with nitric acid and just weld away.
if going new then look at the prices for servicing and parts etc, the 2 stroke is still king of the bunch.
power to weight the 2 stroke is still also the best by a long way.
I would go e-tec for fuel consumption, but the big black merc 2 strokes are still the best option all round.
you can find a merc part anywhere, and the 2 stroke merc Is still very easy to work on.
as soon as you get into the newer 4 strokes the servicing has to be done at a dealer and that going to be expensive, very expensive.
personally I would get the old engine repaired and rebuilt, 2 grand will completely rebuild the powerhead with lots of change.
the engine will outlast you if you look after it. just my thoughts on it.
corrosion eating at the exhaust is a bit weird, but no excuse for not simply flushing an engine out.