I believe jfiore is correct - and that actually may hold true for all Pantah engines. Compared to the newer engines, which have a spring clip that essentially wedges the arm into place on its rocker, the older Pantah engines aren't quite as enlightened. Not that it's a huge deal, and if you haven't worked on a newer engine, you won't notice. But after doing this valve service, you'll notice should ever you work on a newer engine.
Actually, if you're feeling saucy, I think ProItalia out in California sells a DVD detailing the valve service. A friend of mine has the older VCR tape version in which they actually take an aside and demonstrate with a Pantah head (off the bike of course, so it looks like a piece of cake).
Otherwise, first time through this whole process? If you're mechanically inclined, done carefully, I'd set aside at least three hours to get everything apart; if you're not so inclined ... much longer.
In regards to the half-rings on the valves, you may consider replacing them with a collet set from ; it'll run you about $150USD and it should extend your service interval from 6k to 18k (according to them). The nice thing - you can call and speak with Martin himself; he's a great guy - very helpful and pleased to speak with anyone.