Before you do anything, determine what type of oil is fouling your clutch. Wipe onto something clean one of the drips from the weephole on the underside of the bell housing. Sniff it. Does it smell like transmission oil or like engine oil?
If it smells like transmission oil, the most likely culprit is part number GU14085900, the clutch pushrod seal. If you use the stock pieces, you will need two of them. A better seal, IMO, is the stack of o-rings the shop I work at assembles. The pushrod seal can be changed without removing the engine or transmission from the bike. Undo the swinarm spindles and roll back the swingarm and wheel for access. Then, you can flush the plates with mineral spirits by blocking the weephole below with a whittled stick or duct tape, pouring the spirits through the timing hole, starting the bike, and revving it gently while you feather in and out the clutch lever. Then drain and repeat at least once.
If it's engine oil, check the big breather hose for cracks or a loose clamp. If that's all good, you're going to have to pull the engine. Change everything while you are in there. First, the gasket for the breather, GU12007600. Use sealant along with the gasket. Since the main seal has been tried before and didn't fix it, either the seal wasn't replaced properly. I'd recommend pulling the rear main bearing so you can totally properly install the seal (GU90405367, get hte real thing, which is viton) and replace the gasket (GU12001800) underneath. To do it right, you will need the puller and then the two-piece tool to seat and install the rear main seal. I can lend you mine. Use sealant on the gasket. Also, use solvent to clean the threads of the two lower mounting holes that were drilled through into the crankcase. You will need to clean the bolts, too, and then use thread sealer on those two only. Finally, clean the gap around the welch plug up by where the breather attaches. Blow it out with air, and then goop sealnt into the gap. I use JB Weld for that. Also, change out the trans input seal (GU90403548). Get the real Guzzi seal, which is a waveform type.