I had my 98 Tip fail on me a couple/few years ago. Blew a seal.
Did a lot of research finally finding Specialized Porsche, LLC in Riverside, CA. Sam is the owner... and he a great guy. Check out his litle website.
He removed and replaced, but sent out the tran ny for a complete rebuild. Long story short... it's been ~2 years and tran ny is running stong. Total for the rebuild including R&R was ~$4,200. If you send him your tranny likely it would be less than that. 1 year/12K warranty.
Good luck.