1971 ski doo nordic 399

my 1971 ski doo nordic ( it looks rough but dosent run like you think ) that i got for free from a friend, it has a 399cc motor that purrs like a kitten, just bought it new spark plugs yesterday, put them in and it fired up instantly, cleaned out the carb, i also bought it a 1969 ski doo nordic 371 parts sled with a seized motor, i basicly only bought it for the recoil but it dident come with one but it wasent just a pull start it was electric start so i pulled the motor took out the starter and gear, pulled the motor out of my 71 put the electric start on it and it works perfictly and it goes through powder like nothing ...... oh ya does any body know how to patch a plastic ski doo gas tank on a 71 nordic that won't hurt the wallet too bad..