Block off the pump and be sure to block off the nipple on the carb. That washer is supposed to be bent over to keep the nut from backing off.
now look what you did you made me say nipple, bent over and nut all in the same post.
The only big bore kit for the blaster is a 240.
The cheapest way to go is send your top end out to Ken Oconnor and have him bore it and match a new piston.
http://www.kenoconnorracing.com/