I am going to rebuild my 670 first thing this fall.
I am going to take it to the local Ski Doo dealer.
It has crank and rotary valve issues.
Is there any advise as to improvements I can
specify to the dealer to improve preformance

Start by getting the "Issues" resolved first. It's far better to start with a good solid foundation than throw a lot of money into a grenade. The 670 responds very well to tunnel porting & case to cylinder matching. Shimming up the cylinders and matching the CR quench nets gains also. I would send the entire mill to a reputable Ski Doo engine specialist for a performance rebuild. :)

Those twin pipe 670's are pretty close to edge already. Leave engine stock and concentrate on clutching.

Krusty...did I read your post wrong? could you define your interpretation of stock for us. Because I'm thinking that you don't mean completely stock. I've seen too many of your posts. I admit that I had trouble getting twin pipes dialed in for more than 2 days in a row, but there are several improvements that can be made without sacrificing reliability. Personally, I'd stay with a PSI single. I'm with thumbdoctor on this one.

IMHO, I don't think the 670 was on the edge at all. There is a lot of hidden HP in that engine.

What makes the 670 or 583 close to the edge as far as reliability? Is this because of the motor spinning at higher revs? I had a 583 with twin DGs that ran great. No problems with it. Rebuilding another 583 now to put in a Polaris chassie and am going to run twin Decker pipes. I am installing a new crank, top end was fine. Should I, while I am rebuilding put new rings in it? Any reason to/or not to put them in?

idooski; I'm referring to the 95-96 Mach 1. Factory twins, 44's, 501 valve, cylinders raised with head cut. Not a lot of room for improvement. Unlike the single pipers in the S chassis. I assume that is what alscool is working on judging from his picture.

I didn't think you meant quite stock. I like the engine you just described. That would be a very reliable ride. There is some porting that could be done along with some fine tuning of the rotary valve to add a few HP and make her run a little tighter, but what you mentioned is the way to go.