right ive liberated this one from my hacking smashing brother the problem is it needs an air box but he took the origenal one out with a hammer getting one in is going to mean taking the engine out which i really dont fancey, so what are my other options and what sort of costs are involved also can anyone recomend something to seal the tank ive taken the rust out but there are a few pinholes in my welding (when i say pinholes i cant see them the tank just drips petrol) would like to put this on the road asap so i can enjoy whays left of the summer.

To be honest, i'd want to make sure that the tank is welded correctly, I really wouldn't take any chances with it.
However, there are epoxy based sealants available which go on quite thick. I used it on my Lambretta tank to stop the inside rusting. It used to have a lot of issues with the carb, little bits of rust blocking the jets etc...

welding a fuel tank is very risky business. I've seen tanks explode even when they have sat empty for years and filled with water whilst welding them. Most proffessional welders wont touch them anymore. Take a look at this it works fantastic if you follow the instructions to the letter.
http://www.frost.co.uk/item_Detail.asp? ... &subCatID=

yeah ive heard from my dad about the risks but if your careful and you flash the tank then its usually ok, i dont do the filling with water thing as it provides a smaller area for any vapours or residue to collect and then if it goes bang it does it quite well, although i have had one take off from under my foot whilst flashing it should really have checked it was completely empty before i flashed it rather than relying on someone else's sayso, did blow all the rust off though! this is the first one ive had problems with guess my lucks running out. thanks for the info.

ok so then fuel tank sorted but does anyone know what i can do about the air box or does anyone know of a mnfr that does a bolt on filter jobby that i could use? or am i looking at getting some filter socks (is that what theyre called?)

I'd be inclined to wait for one to come up on ebay. Just keep an eye out for anyone breaking a bike. There seem to be a few more GK71's about that the later models.