Is the silvertop engine hard to come by? And would I be better off putting an st170 in instead?

don’t have much of an idea about wiring and ecu. Will I need to solder all the newer connections to fit the silvertop/st170 to the old wires in the mk3? And am I going to need an ecu for the engine to work? This part seems quite daunting to me.

cheers 👍🏼