Hi Ian, You don't need any special tools for adjusting the valves in the 660 engine. The lock nuts will require a 10mm ring spanner, and the adjuster has a 3mm square end. As 3mm open ended spanners are quite rare, you could make one out of an old B.A. (British Association) spanner filed out to 3mm. Alternatively, as the adjusters aren't very tight, you could make a spanner by filing a 3mm wide slot in the end of a strip of steel. Most people I know just turn the adjuster with a pair of narrow nosed pliers, if you're careful it should be OK.
I've used a useful valve adjusting tool bought from a German firm called Kedo.
http://www.kedo.de/ They do a lot of bits for Yamaha singles, adjusting tool
(Spezial-Ventileinsteller) is part no: 30220, price 9.95 euros. Worth buying as it makes the job easier and avoids damaging the adjuster.
On the Skorpion, adjusting the inlets is easy enough, but the exhausts are a bit more fiddly. There's not much space and some people drain and remove the radiator to make it easier. However, it
can be done without doing this, but you'll need to bend your feeler gauge 90 degrees to fit it in. I also cut down a 10mm ring spanner to make it easier.
I'm not sure how easy this is on a Mastiff or Baghira. (No doubt someone will let us know!)
So to answer your question: you'll need a short 10mm ring spanner, a 3mm open ended spanner (or equivalent), and a set of feeler gauges. The jobs a bit fiddly, but it'll be easier next time!
Norman
http://www.mzskorpion.net