sturleyau - clarke should make a bigger tank for the xl250: http://www.clarkemfg.com/
if they don't have a specific one there is most likely one from a similar honda that you could easily adapt on
doublebit - for the 175 gasket kit you can try http://www.maccycles-parts.com/servlet/StoreFront
. I just ordered a kit for my xl185s, they don't show the 175 on their site but maybe they can get it. i noticed some of the kits (at least on denniskirk) say that it's a complete gasket kit in the title and show a picture of a complete gasket kit but in the details say that the crankcase gaskets are not included, something to be careful of.