i have 1.6 8v and 1.7 16v boxers and..
sometimes think why not to convert 8v to 16v also. i even have another spare 1.7 16v engine block with pistons, but..
few things to find first (some parts are expensive and extremely difficult to get):
1.7 16v ECU / door locks
idle air distributor with expensive like ferrari idle valve
1.7 16v Intake manifold with Throttle Bodies!
exhaust manifold (primary diameter is different if i am correct)
1.7 16v throttle linkage.
1.7 16v both heads!
main belts / idlers / tensioners
fuel injectors are different
starter? spark distribution?
flywheels are different, (clutch too), rpm sensor (1.7 ECU) will not understand 1.6 flywheel (different tooth position/crank angle)?? (need to investigate).
i did not hear if 1.6 engine block (84mm) can be bored to 87mm diameter safely. so.. you need engine block and pistons also
want to use existing block with 84 bore? maybe.. do not know, if piston/valve clearance and compression ratio will be good for ride..
of course you can fit 16v heads only and see how 1.6 ECU will work, if spark advance and clearances / timing will be ok - engine maybe will breath better with 16v than with 8v heads.
but isn't it toooo many troubles???
much better to get whole car as full working unit
all you can use from 1.6 engine is Crankshaft (1.6 & 1.7 stroke is 72mm)