Cable ties should work fine. They are used similarly in automotive
applications. Normally, auto's have under the hood temps somewhere in the
high 100's to even low 200's. Assuming you have wet exhaust, you "under
hood" temps would be maybe 150. Remember, marine thermostats are usually
140-150 as opposed to 190 or so in some auto's and cars have very hot
exhaust systems that put a lot of heat under the hood. Plastic items like
cable ties should be good for at least 200 deg. F. If you are worried, use
some hefty ones, about 1/4" wide or more, this are very strong.

--
Ron White
Boat building web address is
www.concentric.net/~knotreel