John: Your situation doesn't sound good to me. The fact that one cylinder is acting different than another pretty much narrows it down to a problem with (1) low compression (which can have more than one cause) or (2) an issue with the ignition's secondary --spark plugs or spark plug wires. Compression can be lost because (a) the new head gasket is leaking (did you check the surface of the block and head for flatness when you changed the head gasket?), or (b) the rings aren't sealing, or (c) one of the valves is sticking. The exhaust valve is the first candidate for sticking, but the intake can stick too.
Heat buildup can promote mechanical failure. When you changed the head gasket, did you remove the shrouds and blow out all the dirt, grass, and crud from around the cylinders and block? It is recommended maintenance.
I'm not an expert, but those are things I would check. If the problem is fuel related, either filter or carburetor, it still doesn't account for the fact that one cylinder is firing correctly, and the other one isn't.
Your engine looks like a V-Twin, an opposed cylinder engine can run fine on one cylinder, it simply lacks power. I'm not sure the same is true for V-Twins. Others may chime in with more wisdom than I.
I don't think its time for a zero-turn just yet, unless that is what you really want. If you can verify that the problem is mechanical, and therefore costly, you might have sufficient justification.