'96 Geo Metro Clutch Slip

My friends Geo Metro clutch was slipping only at highway speeds. I removed it and found oil on the transaxle side of the disk. The disk itself had little wear, and the pressure plate & flywheel surfaces looked fine. It appears the oil is coming from the input shaft. I see no seal on the outside, so I'm guessing it must be on the inside of the transaxle (not too handy). I'm wondering if special tools are required in order to replace the seal. The transaxle is quiet and shifting fine.
Thanks