Difference between 1.6 and 1.8

What is differnce in 1.6 and 1.8 engines the piston is same size,the difference is in the stroke. What must I change to have 1.8 the crankshaft or the connecting rods?

Bore and stoke are different. So, the block is different. There's a few other details too.

Head's different. Timings different. Power deliverly/torque characteristics are different (1600 more revvy and racy, 1800 more low down torque for easier driving)

different bore and stroke. you cannot use the 1.8 pistons with the 1.6 block as the heads are different. the 1.6 head is flat faced. if you want a 1.8, use a complete 1.8 instead of boring and stroking the 1.6

I asked about the MK2's 1.6 and 1.8 in the Haynes manual it says that the bore is same for 1.6 and 1.8 81mm

the mk1 1.6 were 1588cc, the mk2 were 1595cc. the mk2 1.6 had the same bore as the 1781cc (81mm) but i believe the stroke was 77.4, instead of 86.4
 the mk1 1.6 had a 79.5mm bore and a 80mm stroke.

Are the front part of the exhaust pipe the same (the bit that fixes to the manifold) thanks

Toilet bowl flange is the same between the diesel and the gassers, that is 1.5D and the 1.8.

The single point exhaust had 2 different sizes and if you convert the 4 to 1 manifold, to a 4 to 2 from like a Scirocco then there are 2 different styles there as well.
IIRC.