Consider whether it's worth it:

A14: 80 hp
A15: 80 hp

The A15 has the advantage as it makes more power than A14 at low RPMs (it has a broader torque curve).

A15 rods are identical to A14 rods. The pistons are different. The blocks are identical late A14 and A15, but the A15 oil pan is notched. Early A14 blocks can be notched, or the A15 crank throws machined down and heavy metal balancing employed (to avoid having to notch the block).

It is highly recommended to obtain an A15 if you want the increased displacement. It's less expensive -- if you can find a good running A15. Make sure to get the 80 hp version as 67 hp A15s are common, e.g. from Vanette (or in USA from 1981-1982 B310).

Or better yet, keep it all A14 but fit 79mm pistons. This gives more displacement than an A15. Torque and power are similar. The same can be done with an A15 for a total size of 1608 cc.

Forged pistons are not recommended for a street car as they are less efficient and noisier, however they are needed for sustained high-rpm use (9000 RPM) or for super-high boost levels.