Are there any stoker engines out there for any Porsche models?

 

LN engineering sell a bore kit and you just need to find good tuner and good advance mechanic.
you can go to 4.2L

 

I believe a stroker engine uses a modified crankshaft to allow for greater piston travel. A bore kit will make the engine "over square".

 

Correct, you are increasing the stroke of the crankshaft.
Some extremes even require the case and rods to be shaved to allow clearance.

I once had a vw beetle with a crank that was stroked from a stock 69mm to 82mm.
This provided tons of low end torque.