Horizons Unlimited - The HUBB - KLR UK/Europe parts suppliers. 2002 KLR 650 C model Burning lots of Oil

I have a 2002 KLR 650 C model with the following problems:

· Burning a lot of oil, almost 4 litres in a month of riding 70 km per day, mostly dual-carriageway and city traffic, then off-road spins at the weekend with plenty of steep hills.
· Smokes a lot on startup, I think the smoke is fairly dark.
· Cuts out and refuses to idle, unless I set the idle way to high.
· Backfires very badly when off the throttle and if combined with a steep down hill, she blows plenty of smoke out the exhaust as well.

She has had a hard life with me, was in good nick when I got it at 15,000 km, but I have done 25,000 km in the last year and some of those were on a trip to Morocco with lots of motorway in between.
I changed the airfilter and there was marginal improvement. I haven’t done anything to the carb which I suspect is dirty and causing the cutting-out.

But because of the severe oil burning and her hard life, I suspect it is engine trouble. I have never had the valves checked, only engine work was replacing the doohickey.

I am going to bring it to my mechanic for a compression test and hope that this will help me decide how to proceed.

Suspecting the worst can someone please tell me where I can source a new piston and rings for the bike?

Has anyone any experience with Oemmotorparts?
http://www.oemmotorparts.com/oem5.asp?M=Kawasaki&T=KLR%20650&Y=2004&L=KA-KLR650-4&O=Cylinder-%20piston&F=KA-KLR650-4_AC&L2=KA-KLR650-4_AC

In the US they have a 685 bigbore kit that is highly recommended but apparently there is nobody in Ireland that can do an engine bore-out.
Should I just go with the OEM parts?
Anyone know if the piston and rings are common on the A and C models, say from 2000 onwards?
Is there anything else that should be replaced while this work is being done, valves, valve seals?

Any comments, advice or links to UK/Europe parts suppliers would be much appreciated.