For AN fittings, a lot of people use G and J in Ontario.
http://www.socaljetboats.com/engine-mechanical-electrical/steel-braided-line-20839/msg237170/#msg237170I'm sure there are places closer to HB, just look up steel braided hose or AN fittings.
Probably about 98.75365976% of jet boats don't run a thermostat. If you really want or think you need one, do it after your overheating problem is dealt with.
When your boat is running, do you have a strong stream of water coming out of the dump line?
Is this something that just started happening? Does it overheat quickly after you've launched?
The build up on the gate valve "might" be enough to cause the problem. Is the valve on the jet or the dump line?
Sometimes sand can build up in the water jackets around the cylinders. There should be some plugs (3/8 pipe I think) on both sides of the block. You can take those out and run water through the block and flush out any sand or debris. Don't let water go to the headers.