Hokay, so I did some digging around in my garage and found the left over rings from a couple of 900cc and 1000cc engine jobs.
Hastings has several available dimensions for overbores. Check to be sure when ordering or sizing from Hastings site (which is EXcellent). With used bores (ie: not perfectly parallel - common with the 900 and 1000cc iron lined barrels, not so apparent on the 750's and almost unheard of on the 600's: they don't make enough power/heat to wear much) it's a good idea to go one overbore size up on the rings and then carefully file the rings to get the required end gaps. BE SURE to get the end gaps right for the ring material you select. ie: cast rings require larger end gaps than chromoly rings. Check the mfg'r specs for end gap which is usually provided in 0.000" per bore inch. eg: 0.003" per bore inch would be approx 0.003" x 4" of bore for a 1000cc airhead.
I think I paid less than $50 for these rings and, of course, have enough to do two engines.
For the 900cc airhead motor, hastings part #5562 for a 4cyl Saab will work. Top compression ring 1.75mm, second compression ring 2.0mm and oil scraper is 4.0mm. After accurately measureing your bores check the site to determine the exact ring size you need to accomodate your bore's "special needs". They're available in incremental sizes to suit overboring etc.
For 1000cc airhead motors I've used Sealed Power (Federal Mogul) cast rings for a VW bus. Part #E-495X 5594. The cast rings work well in nikasil bores as they're soft enough to bed nicely with a well prepaired nik bore. Chromoly rings are so hard they often never seal entirely before the bores glaze over.
I install rings completely dry and fire the motor and run at 3/4 full revs for 45seconds then shut it off and repeat twice more allowing it to cool in between. Then suit up and leave on the bike immediately after starting (DO NOT let it sit on choke to warm) and turn the choke off AS SOON as possible (or sooner) while underway (this is very important). Go for a good ride of 40-60 miles of empty roads using the revs in the midrange off and on throttle to create pressure in the chambers to force the rings to bed-in nicely to the bore. Your motors will seal nicely and won't use oil when the rings are broken in this way. Don't be shy with new rings, it'll only lead to glazing and oil consumption. You're actually completeing the machining process when you bed-in new rings. Be aggressive about it.
Hope this helps,