You may be lucky to find a gasket, they have been out of production for many years now though.
Does it have good consistent spark? Not impossible that it is the coil itself. A quick google says 10-35 degrees btcd so that looks ok.
Personally i would have started with a compression test, than a leak down test to rule out mechanical. one other check is valve timing, easiest test for that is at TDC (not comp stroke) the both valves should rock.
it may seem simple but have you tried a new plug?
