[Henry Schmidt]

First, turbo power on the track is great fun but generally means increased cost of maintenance. Weight the cost before proceeding.

If the damage to the 964 crank is a spun rod , they damage can usually be repaired by simple grinding the crank to the smaller 53 mm journal used on the SC and GT3 cranks.
The crank shop can grind the journal with a large radius which will greatly improve torsional rigidity of the crank. At this point you can run a 3.0 SC rod, a GT3 Pankl titanium rod or a custom aftermarket rod. Charles @ LN makes such a rod and may have it on the shelf.

Net you can buy a GT3 crank. They a ridiculously inexpensive and are a great choice for your application. You only need to buy a custom crank pulley to make this crank work in a 964 application. Richard Clewett @ Clewett Engineering makes the pulley.

Whether building a turbo or NA engine bigger is generally better.
You've go lots to think about so way your options carefully so you only build the once.