I have an 87 t2 with a bad S4 motor in it. How hard would it be to put an S5 motor into my car? What are the main differences in the motors, and would i have to change anything?

 

The S5 TII engine has slightly higher compression (9.0), a slightly revised intake manifold, much better turbocharger, electric metering oil pump, thicker rear iron casting, high impedance injectors, slightly different spark plug locations, and a few other little differences.

If you do swap the S5 engine into the S4 car, it is best to just swap the block itself and turbo. Use the S4 intake manifolds and electronics. Swap the S4 front cover on with the mechanical metering oil pump, which I believe also requires you to use the S4 water pump housing and pump assembly.

Besides that it is straightforward.