At 102k miles the clutch (if never changed) is probably worn out anyway.
Oil can leak onto the clutch from the crankshaft rear oil seal, sump gasket or gearbox input shaft oil seal - you won't know for sure which is responsible until you have removed the gearbox. But your deduction that it's likely a leaking crank rear oil seal is possibly correct.
The crank rear oil seal can be easily changed after the flywheel has been removed.
1. Note the position of the flywheel so that it can be replaced in the same position. Iirc, there is a small dimple mark indicating the top dead centre position (like a small drill tip mark near the edge of the flywheel close to the ring gear). If no mark can be found, mark the flywheel and don't turn the crank while the f/wheel is off.
(Edit:- The crank can be prevented from turning by either of the 2 methods described at the end of this guide)
2. To remove the seal :- some people just prise it out using a screwdriver, there are also proper 'seal removal tools' (like a lever with a hook on one end). It's important not to damage the sealing surface on the crank. One easy way is to drill a small hole midway between the inner and outer diameters into the body of the seal (there's metal under the rubber surface), screw in a self-tapping/sheet-metal screw, then use a claw hammer or pliers on this screw to prise out the old seal.
3. Clean the sealing surface of the crankshaft where the oil seal will make contact. Often rust will have formed on the part of the crank that is just outside the seal - this needs to be cleaned off. Check that there is no ridge worn into the crank where the seal makes contact. I generally use fine wet and dry abrasive paper to clean up the crank surface. Then clean the crank surface and the seal housing.
4. To fit the new seal:- apply a film of oil or grease to the crank rear surface and the sealing lip of the new oil seal, carefully position the new seal the correct way around, (the sealing lip surrounded by the garter spring faces inwards) on the rear of the crank. then gently and evenly tap it home using a block of wood and a hammer. It's important that the seal lip doesn't get damaged as it's fitted onto the crank and the seal is inserted without being distorted. The seal will need to be driven in until it's flush with the housing.
Then clean off any oil from the rear of the crank, refit the flywheel (clean and apply thread locking compound to the bolts) and torque to specifications. To prevent the flywheel from turning as it's being tightened, either screw in a bellhousing bolt and wedge something (e.g. a lever/ screwdriver) between this and the flywheel teeth, or clamp a vise-grips onto the ring gear to prevent rotation.
Hth,
Al.
