AAAAARRRGGGHHHH!

That is low compression. Likely, the bike won't start. First thing to do is figure out why the compression is low before you take the bike apart. If the low compression is not caused by tight valve clearances, then the valves are the problem or the pistons/ring seat. Remove all the plugs. Put a teaspoon of oil down #1 plug hole and retest the compression. If the compression jumps a BUNCH... say 150 psi, then you can be sure that the piston rings are not sealing on the cylinder walls.. if the compression doesn't jump a bunch, the cylinder head is the likely culprit.

If the cylinder head is the issue, buy another from eBay and hope it is in better shape than your current head. Anything from a 73 to 80 Z1/KZ head will work (903/1015). I would disassemble and lap the valves in and check for seal. You can ask about that in a separate post. Not hard... all it costs is a new head gasket and the new head.

If it turns out that the rings don't seal, you can't know why till you disassemble pull off the block (and the head first). The gist of the issue is to measure the cylinder bore to piston clearance. That is, measure the piston diameter and the cylinder bore diameter and the difference will tell you if the distance from the piston rings is too great to allow a seal. If you do the measurements per the factory shop manual, you will also know if the cylinder bores are oval (out of round) and if the pistons are worn out. One of the easiest fixes if you find the rings aren't sealing is to buy a very low mileage block/piston set... have the seller mark the pistons for which hole they go in. Most of the time a new set of rings will get your compression right back up after honing the bores... this can cost less than $100 plus the gaskets.