A timing belt which drives a camshaft has a lot of alternating load. If you are going to make your own toothed belt, the major difficulty I envisage will be the positioning, and tensioning, of the reinforcement. This is needed to resist belt stretch, and might be steel cable, Kevlar, carbon fibre, etc. Any hint of stretch in use and the teeth will no longer fit the pulley, with unfortunate results.
In my book, there are some things which I will not even attempt to make. Sparking plugs are one, and toothed belts another.
Not good news, perhaps, but better than finding out as the engine blows up.
Cheers, tim