As long as you're willing to spend the money, it can be a great ride. One of the first things if you want to do it right would be to split the case, see if it needs a line hone or more serious work for the crankshaft bearings.
It should have the appropriate timecerts and case-savers to reinforce the soft magnesium. It would also be advisable to do the oil bypass modification and get a later oil pump.
Going to 2.7 would require a little bit of work to increase clearance for the slightly larger cylinders. With a good set of heads and the right camshafts it would give a good horsepower boost.
You will also want to get a front oil cooler setup. The stock engine oil cooler is marginal and won't do enough if you bring up the power. I installed a later SC oil cooler setup.
I would recommend getting in touch with Henry Schmidt
https://forums.pelicanparts.com/members/21558.html
He has many years of experience bulding Porsche motors. I have been keeping my late model '73.5 CIS mostly original. The numbers matching curse stops me from doing much, aside from putting in an '86 3.2 Motronic engine for a few years. That only required a few easily reversible modifications. I do miss that engine, but my stock 2.4l is still a lot of fun.