There are several big end shell bearing sizes colour coded to the Crank shaft inner lettering and the Con rods to be used(See the manual).Some of them are no longer available.You would be better served getting a decent running engine or a decent matched crank and rods.
I have postulated that the thickest,"Black" colour coded shells could be used on a worn crank but should be checked at a local engineering/engine specialist who have the correct equipment.
There are no official undersized shells for crank regrinds on these engines
The good news is there's usually a lot of decent low mileage sets over there in the USA.In fact the crank in my Present CX500 here in the UK was from a USA Ebay seller/bike scrapyard.
I bought from the US as I know the CX/GL500s over there haven't had the abuse,in most cases,that the UK CX500s have had and at the time the Pound/Dollar ratio was in my favour.
I was also lucky enough ti be able to get all my shells over here but had to source them from a couple of well know reliable bike supply companies.
You can make inquiries at,
http://www.davidsilverspares.co.uk/
and this bloke knows virtually everything about these bikes/engines,
http://www.johnoldfield.co.uk/
and both companies are great to deal with.If you contact John Oldfield he can advise you further.
Note:All CX/GL 500 crank shafts will go into all Crank cases without the need to replace the Main bearings.I've had some of the worst abused engines you could imagine but even on these engines the Main bearing white linings are still perfectly usable.