RE: Water cooling setup?
I'd go with one to the waterjacket, in low, out high and overboard and the other to manifold then pipe and out.
You are going to be cutting that exhaust header down a lot before you find a tuned length so don't plan your exhaust mounting hardware at that length. You will be looking for 320 - 370mm from exhaust flange to widest part of pipe measuring down the centre of the pipework.
Are you a member of [link=http://smbc.se]Stockholm Model Boat Club[/link], lots of good help and a friendly crowd.