Hi 2valve,
like the others said, a parts list might help. What are you after?
Seeing as though you are in Australia, there are some second hand sources available. Melbourne has a good airhead parts guy who posts stuff (PM on it's way). I've got a fair few R90 bits and bobs. Like already mentioned second hand from German ebay is also great.
For new parts, I think you'll find that Germany is the place to buy for us at the moment. I've spent a lot of money with Motobins last year, who are great to deal with, and have an easy to understand catalogue, but we are just getting a lot more for our dollar with the Euro instead of the pound right now. (don't forget that most German quoted prices inclued 19%VAT which we don't have to pay). The German sellers are also great to deal with.
There are a lot of German airhead parts sellers.
http://www.siebenrock.com/,
http://www.motoren-israel.com/, I'm about to use
http://motorrad-stark.de/, and I know there are a lot of other ones I have stumbled across in the past.
I didn't quite understand what you meant by the two parts that you need. If you need to buy new barrels and/or pistons, it might be worth looking into the siebenrock aftermarket stuff- their power-up kits. Depending on what side of September 1975 your R90 was made, there are two full kits for the R90 (1000cc nikisil barrels 9.5:1 CR, lightweight graphite forged pistons, lightweight gudgeon pins, and circlips and new rings). You get an increase in power and torque, and a serious update in technology. The later R90 kit looks like it costs about Aus$763 (from Siebenrock) before postage, the earlier R90 kit, about $835.
Not cheap, but if you can even find brand new stock stuff, neither is that.
I've got a kit for my G/S that will go on it soon- very nicely made stuff and reports from long term reliability are very good.
Anyway, good luck.
) where's the best place to purchase ? munich /moto bins / moto works ? have read some posts recently regarding a " german" contact " for spares ! . at the moment im not really sure as to what exactly i'll need ( well i do know two parts ,set of STD bore barrells which i do have the first size over size pistons to suit ) was trying to have the contact details when the time comes to purchase the parts. any advise will be much appreciated 
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sitting on carpet and coverd over). the serial number / chassis number is 4083164 year 1975. the top end was recently opened up and found some light rust in the bores, the bm guroo honed the bore but he's not happy so im not happy, i like things done properly ! im one of the guys that polish his lawnmower 
, so atleast you know how sick in the head i am 
thanks ontic for the info and the PM ( very much appreciated for that, will reply with a PM also ) regarding the top end big bore kits, sound like a very good option and no more rusty bores. really would like to keep the bike std if possible but considering it rarely sees daylight the nikisil is the way to go. but it depends if i can find a clean set of std bores to suit ( already have the pistons etc which is 1st over size ) regarding about the other parts i may need i'll hopefuuly receive a call next week with the list and i'll pass them on just in case for some more advise.