[Nasher]

Unfortunately as you’ve suggested the oil could be coming from several sources:

Above from the valve seals
Below from worn bores or incorrectly fitted/worn rings.
Sideways from a head to barrel issue. Our engines have no head gasket but do have ‘O’ rings at the joint with oil passing through them.

My first instinct was that it was valve seals, but It sounds like you are using quite a bit of Oil and that could point at either of the other two.

Which cylinder is it that the fouled plug comes from?

A compression and/or leak-down test will establish if the rings and bores are worn, but only if the valves themselves are sealing properly on their seats.
I honestly don't believe you'll hear air escaping and establish where the problem is.
Have you checked the valve clearances?

If it was mine I’d be whipping the heads off to have a look at the ‘O’ rings and give the heads a quick refresh.
The only special tool you’ll need is one to torque down the head nuts afterwards.
The vertical one is a faff but achievable I’m told without removing the frame on a Monster.
I’ve only done it with an engine on the bench.

Inspecting the joint faces will immediately tell you if you have a leak.

With the heads off it’s an easy job to change all the valve seals, and a look at the wear ridge at the top of the bore will give you an indication of bore wear cylinder to cylinder.
The valve shims and seals are far easier to do with the heads on the bench so it’s worth doing them after lapping in the valves.

I’m not actually sure if the barrels will come off with the frame in place, but again if it were mine I’d whip them off, hone the bores and put new rings in.

If you don’t want to lift the barrels, once the heads are refreshed and back on the engine, spinning it up and doing a compression and/or leak-down test will establish the condition of the rings and bores.

Good luck

Nasher