If you have a stock front(13) sprocket then you could use a 36 rear sprocket for the same gearing as 15,42, If you have a stock engine the 15,42 or 13,36might not pull very good and bog in your upper gears. But the only thing you might have to do if you do decide to go with 15,42 is add a link to the chain.
how fast will u think i will go if i have a 42 teeth sprocket without a 15 teeth front and with......

how much will a link cost and how hard is it to put on
you could just buy another master link and run two. they're under $5.00. If the motor can pull you in 6th gear you might do 60+. Thats pretty though out of a stock blaster motor. They are only 17-18 hp stock.