The "shims" are just that shims that are fitted into the top of each bucket that sits on top of each valve, the shims are varying thicknesses so the gap between them and the cam is correct, if not changed/checked/adjusted for a long time, and most aren't, it can get very tappy, this affects power also, as it stops as much air as should be going into the engine and as much exhaust being pushed out as should be as the valves don't fully open.
The tappets are easy to do, just need basic tools plus feeler gauge, and vernier, they are a VERY common shim, used on seat/vw/pug/citroen/renault etc etc.
You can even do swappsies, ie if after measuring the gap is too small on one valve you can use a thinner tappet off another valve etc, they are about £3 each, so a whole set would only cost £24 anyway, take about 3 hours to do, no need to remove cam even to do them, they can be flicked out with a decent screwdriver levered against the cam.
The best thing is to buy Haynes book no 1172, this covers your engine in detail(and the 1.6), it is a engine only manual, but well worth the money.
tabetha