Varying spark, out of ideas

Not the same timing problem but,, I had a similar low compression on a slant. The previous owner installed the timing chain 1 tooth off and drove it for years that way. Really rich and really low power. The rings, valves, etc... had tons of carbon build up. A LOT of blowby.

After fixing the t-chain, this isn't an add, but I happened to watch a YouTube video of a dude using Berryman B-12 (similar to seafoam but better in my opinion) to get his sealing rings unstuck. I put some in thru the spark plug holes, let it sit, some more in the crankcase, ran till warm, cool, ran till warm, then changed the oil. It actually worked. Went from 100-110 to 150 average and the blowby isn't an issue.

Not saying it will definitely work for you. If the rings are indeed worn badly then rebuild for sure. 1 can of Berrymans is worth a shot though.