The owner manual for the GXV140 and GXV160K1 specifies inlet clearance 0.15 mm (0.006") cold, and exhaust 0.2 mm (0.008") cold.
I have not been able to find a mention of a timing mark in the manuals. You can find TDC approximately (within plus or minus a couple of degrees) with the pencil-in-the-sparkplug-hole trick.
It sounds from your description as if the first thing to be concerned with, is the valves and their sealing. If the valves are not closing properly you will probably find inconsistent tappet clearances, each time you run it.
