Cooler rack: check on EBay... when I was looking for a new rear grab bar I kept seeing an aluminum one with the six pack holder (might be too small?) for like anywhere from $25.00 to $79.00 not sure on the $25.00 one though as I emailed the seller a couple times for sizing and he never responded so... let the buyer beware on that one...lol
Seat cover... look up a member here called Mosh or something like that, they made my Yamaha old school bar pad that is awesome but has been retired because I friend (another member here who lives in the next town over from me) found an original Yamaha bar pad from the very early 80s at his work so that's now on the trike! But Mosh has done a ton of seats for members here and does customs too.
Gas tank: Ebay or try the classifieds here, lots of members have 250R's on this site so there may be one for sale.
Stickers /Graphics sets: Ebay, but if not, there is a member here, I forget his mane but I believe he does repro stickers, maybe he'll see this and IM you or someone else will hook you up with his contact info.
Choke and Carb kits: Ebay... or try Partzilla . com look up your trike and then check out the
parts lists for the different
parts and part numbers... I got most off the
parts
to rebuild my trikes carb from them. it worked out to be just a touch more than what the kit on Ebay was listed for... which I would have bought but it sold before I got off my butt to buy it... lol