japanese sports auto wreckers (i think thats what they are called) here in bayswater industrial area have an r33, also an rb25 but who knows if they would sell you the oil sender
it will be more a case of the studs for the crossmember not lining up, you'll probably need to remove your studs and put new ones in place for the manual crossmember.
this is what has to be done on an r31 anyway...
(tiesto has a battery light actually on his battery)
You may be right, the alternator could be causing a weak spark at low revs? keep trying things i guess