Time/effort/expense is going to be a LOT more for the engine swap. Assuming it's the "easiest" option, i.e. an EX code 1.8 from a Scirocco you'd still have to do things like unbolt the exhaust, drain the coolant, undo all wiring and fuel lines. You then have the issue of the carb jetting being incorrect (assuming you're not getting the 1.8 carb). If you want to use your current gearbox then the clutch pressure plate has to get swapped too. Possibly more things that I've forgotten.
I'm not saying don't do it - as it's invaluable experience for future conversions (
) but the gain for all the extra work would be 15bhp in theory and more lower down torque.
Cambelt is fairly simple - especially if you use the Fuzz Townshend method from Car SOS - cut the old one in half LENGTHWAYS so you have a half-width belt on there - this retains the relative positions of the cam, crank and intermediate shafts. Slide on the new belt and cut off the remainder of the old one. Job done, no re-timing necessary!