As said the engines internally are different. In short.
The 20v turbo has uprated pistons to create lower compression, a stronger crank shaft, different cams, bigger injectors, different ECU, different exhaust manifold, different induction manifold, bigger throttle body and has stronger valves too.
The turbo runs boost pressure at 1 bar.
the turbo also has, a Turbo, an oil cooler, an intercooler + pipes.
Then the suspension is uprated with Much bigger discs and a longer ratio gearbox with a much stronger clutch.
If you do a conversion you would possibly be able to use a 20vt downpipe plus turbo etc. You would need some form of ECU upgrade and mapping to tailor it to the engine spec.
If running very low boost from the turbo you should be ok with the stock internals but they will be the weak point.
Compression may possibly need reducing, it can be done via a fabricated metal plate to replace the head gasket. Otherwise pre detonation will occur and this can cause severe damage.
Unless you are going to do it properly then you will be running a very detuned turbo say 180bhp and the cost will be huuuuuuuuge.
By far the cheapest solution but still expensive is to transplant a coupe turbo engine. this is still costly and you have to find the best mechanical donor car money can buy. The benefits of this are that you get all the suspension, gearbox, brakes as well.
