Most important is to determine if the cams and followers are bad (pitted). The only way to find that out without dismantling is to have a trained mechaninc (or some expert) to listen to the running engine. If you hear som sharp "metallic" sound from the
heads, the cams/followers may be bad. If so you have to replace or repair them.
Second item is to look for oil-leak from the junction between the cylinders and heads. An oil leak here is an indication to bad gaskets. The first type (only on 81:s) has a habit of self destruction...
Third; the starter. A rather loud device (OK) but it has to funcion correct. The starter engine may be slipping or simply not turning; both items easy to fix.
Then there is only standard items as to look for binding steering-bearing (the bearings are often neglected and has to bee serviced or replaced with conical dittos), see if there is some play in the wheel-bearings (common in the rear wheel).
Summary; the 2 first items is most important and here you can (should) decide not to buy. The others is simply good reasons to negotiate the price down!
Good Luck!
Lars