Or take it out (same covers as a 944, so easy) and soak in some ATF for a couple of days. Should loosen up. Old 944 tuner's trick.
If you think maintaining a 944 (or 951!) engine is a pain, get a 928 - it's the same thing as having two of them welded together!
Change your belts before you start it again.
AFAIK the 928s have interference engines just like the 944 / 951 - bad belts have caused much cursing and throwing of tools after new / excitable owners fired up engines that had old or dry-rotted belts.
I don't even want to think about how much $$$ it'd take to replace 32 bent valves (*shudder*) or did they do away with the interference design on those?