OK first thing first, i have never owned a motorbike untill 9am this morning when i purchased a 1981 ATC110.

The advert said engine complete but its missing tons of parts. As you can see from the photos below.

Can someone please advise when what I need to buy to complete the engine?

 

Those "missing" bits will be in a box somewhere, if the seller can't find them you have had it for that engine. Unless, for originality, you want the original engine, the best way forward is a replacement. fortunately the Chinese are making those engines by the shipload. However there are several different models, kick start, pull start, electric start, and manual clutch, "semi automatic" etc. Also some have only forward and reverse, some have four speed forward only, You could get lucky and find a genuine 110 engine from an ATC being broken for spares, but otherwise find out what clutch/gearchange/starter system your old engine had, and get the nearest you can to it.