So I am starting to work on a 1970 AT1-B. Needs a lot. I notice there is no starter solenoid (one of several parts missing). The new ones I see on eBay (Partzilla indicates NLA, so far) are all pretty pricey (like $100 +/-  ). 

Has anyone ever used one from an XS650? I have a couple of the bikes, and spare functional solenoids. Any reason why one of them should not work? Both 12VDC, not necessarily same mounts but mounts can be made.

Also, the regulator (which is there) looks pretty sketchy. Those are also pricey bits. Anyone using a solid state unit? I see solid state regulator/rectifiers certainly. The AT1 having a generator theoretically needs just the regulator.

I intend to rebuild the crank (probably send to Bill Bune). What suppliers do you use to get the rod assemblies? I see Vintco has a complete setup, as well as the outside main bearings and seal sets.

Appreciate the input! 

In my opinion any solenoid would do the same job.
Only has to bridge the terminals from within when activated.
I've used chinese 'almost free ones' 'with free shipping too' on larger quad bikes with no issues.
Regarding charging, i'm not sure on solid state stuff, but being DC you'll need a reulator with cutout [to cut in & cut out] which is what the original one was.
I'd try your original first even if ot looks sus.
If you have a spare regulator from your 650 i think that would work also as 650's had a brush DC generator by memory. Brushes running straight in onto a flat double slipring thingy?.
By now you'll have gathered i'm backing into a corner lol so best wait for the experts.

If you could find a regulator that was for the same "design" as the Yamaha, you could probably use it.  That said, I'm pretty sure it's not exactly the same as some generators as somewhere I read once that Yamaha made it "self energizing" so that if the battery is low/dead you can still start it.  Some generators require battery voltage to energize the field coils and may not have had the "cut out" circuits Yamaha had like RT325 mentioned?

I think I have a box with at least a couple used AT1 solenoids and regulators. 

On the rod you can still get everything from Yamaha last time I checked.    Very common rod...  all the 125 and 175 Enduros used the same rod from 1969 to 1981.  MX, YZ, and IT 125 or 175 too until at least 1979.

Thanks RT. Quite right about the XS charging system and the similarities. The XS regulator does cut in and out as the voltage fluctuates. I lost part of the wiring harness on a '74 TX due to the regulator sticking 'on' I believe once. Replaced both regulator and rectifier with a single solid state unit on that one. But I do think I have a "good" one from a '77 that I replaced both units on so I will see how that fits up. 

This bike will not be a full-blown restoration, but a rider, so originality or "correctness" will not be a goal.

Thanks again!

The larger bikes used AC generators (aka "alternator") which is why they have a rectifier (diode) pack that the AT1 and smaller bikes with a DC Generator did not.  I doubt the regulators are compatible?

Oh gosh yeah i'm dreaming thanks Mark. Brush type alternator on those 650's not gene's.

Yup. Should have remembered that one myself!