Marinizing an item on an engine involves setting it up to suppress any likelihood of any internal sparks triggering an explosion. Within the alternator there are brushes running on the slip-rings of the rotor. These generate very small sparks as the rotor spins. Nothing can change that, so if there are petrol vapours around there tends to ignite. A marine alternator has mess screens on the open ends to prevent that flame getting out and triggering a wide, more devastating explosion... One of the side effects of the screens is they reduce cooling through the alternator, which is probably why you can't find a cheap one with a high output.
As for a DIY kit... Never heard of one. Possibly because as a DIY it can't be SAE certified.
Just out of curiosity, why do you need 160A? Most of the 'big' alternators for marine are in the 65A area... That seems to meet the needs of most boats....
Chris.......