well the 20v manifold and 16v manifold share the same inner 3 bolt holes, only the outer 2 change slightly. all the ports line up. so you could easily probably make a 16v turbo one work, with a minimum of re-working.
not sure what you're asking about turbo and n/a?
yes its feasible to turbocharge the silvertop. its a 10.5-1 compression motor, however. same death rate probably as turbocharging a b-series honda or similar hi-compression motor. high-comp and boost don't go well together. however a small amount of boost will make a good amount of power. its all a matter of how long you want your motor to last, and how much power you need to satisfy you.
if you just want a turbo 4age mk1 tho, i suggest running a GZE block and lots more boost. i know a GZE should be able to handle 200hp for a lot longer than a 20v would wanna live under those conditions.