Welcome aboard Dave.As per other forums the 650s may start struggling soon as parts are not as readily available as the 500s.I know Hann from the other forums had to swap to a 500 engine when his 650 blew.
As you most likely know there's no standard for re-grinding the cranks on the 500s or the 650s.Dr Don has done it but it's not easy.
Also there's no Piston rings for the 650s.
In way of help though bookamark this,
http://www.srm-engineering.com/page_1540I had a conversation with them a year or so ago and they will pickup a block/rods etc door to-door for around £30 then there's the work and parts costs but and can grind down cranks and Plasma weld them back up the stock size/re-grind to standard so normal shells can be used and they know there stuff.I have mate who does drag bike racing and he rates them highly.
I think I costed it to around £250 inc which I don't think is that bad considerning the level of work.
They may even be able to do re-bores and find substitute rings?
I only mention this in passing and for future possible needs.