If you are 100% sure the stator is OK, but see 18v at the MOSFET RR output, then it sounds like the RR has failed. Should max out at c.14.5V.
Only achieving max 12V at the battery suggests current is leaking to ground/neg somewhere. Have you checked out the wiring loom, connectors, neg/earth points? Do you recall seeing any 'flare' from the headlight as revs increased; maybe blackening witness on the bulb?
Is your MOSFET a Shendengen (good) or a no-name? (not good). Did it come with ready made connectors or was it connected up with crimps/soldered in? I am thinking maybe one of the connectors has been fried internally. Some fitments suggest direct wiring to the battery, rather than via the standard loom - how is yours fitted?
Is your RR in the standard position, RHS behind the engine or was it moved out into the wind, with extended wiring - they can overheat, but should cope with up to 120c ...ish - any more & you may see witness on the rear face.
If the above check out, have you tried another battery in case yours is caput - this was our first reaction to your post remember. Odd things can happen if a battery has been overcharged as the RR would have failed progressively.
Good luck with the gremlin ................... KEN