Maybe it can be of use to someone who wants to build a track car where originality and pedigree doesn't matter but even then the price is too high.
To convert back to 964 you need (just off the top of my head):
New front bumper + centre grille + seals
New front bumper reinforcement bar, brackets & fittings (don't think it's the same as 964)
New headlights + indicators + fog lights
New front fenders
New front wheel housing liners + brackets
New fuel filler housing
I think some of the plumbing inside the fenders such as water tanks, pipes etc will be different for 964 so will need replacement
The 993 rear fenders have to be cut out and new 964 fenders have to be welded in
New rear wheel housing liners
I think the cross member behind the engine will either need to be replaced or reworked to accept the 964 centre reflector
New rear lights + centre reflector
New rear bumper + 964 bumper supports
New 964 heat shields
964 side sills
964 engine lid + mechanism + grille (I don't think the mechanism + grille are interchangeable with 993)
Some of the interior panels seems to be 993 and will have to be replaced with 964 including the front & rear seats & doors
Wheels are 993
Oh yes, then when all is done, a respray.
A very, very, expensive exercise to convert it back to original in other words. Much more expensive than just buying a really good 964. Or 993.