why not start by trying to get the 911's existing engine (and transmission) running -- unless you KNOW it was put away broken? Lots of folks have had good luck firing up engines that sat longer than yours. For good/bad, these are the heart of the 911 experience. Next, if engine no good as-is, check out
https://www.kennedyeng.com/engine-adapters and see whether they can provide you with a kit to match your VW engine to the 915 tranny?