nice to see an interest in a basic bike, very popular though. i had one myself!
i'm not aware of any good, close-up pics and data available but hopefully yours is original and, you can go from there.
depending on what you need....there are plenty of amber-walled tyres around, ebay and so-on. apart from that, shimano sis and other 18 speed stuff should be available from our 'wanted' section. just try be as specific as you can to get the offers coming in.
for me, i tend to buy a cheap bike for it's spares. look around your boot sales, charity shops etc...you will gain a life time worth of spares for little money. tyres were very basic stuff. cheng shin type with little technology in the tread. often known as a centre-ridge pattern.
http://www.ebay.co.uk/itm/Ralson-26-x-1 ... 869wt_1110
type raleigh mustang in the top right-hand corner and let the engine do the work for you. some users have mustangs, i'm sure.
good luck, get some pics up of yours when you get time.