Well, for a lack of other advice, I'll poke my opinion out there. $30 for a FMF header is a good deal and you should definitely run it. Will it work with the stock muffler? probably... Will you be getting the maximum benefit out of the stock muffler? probably not.
A KX85 muffler probably won't fit the blaster the way it is now. The blaster has a deeper dip in the pipe than most dirtbikes. You might be able to make the KX85 muffler fit. With enough work you could definitely get it to fit perfectly but with that much work you could mow lawns until you've got enough money to buy an FMF silencer. If noise isn't an issue, I would highly recommend getting a FMF silencer for it. Any of the versions except for the SST silencer should work. Maybe post something in the classifieds wanted section. Someone probably has a FMF silencer laying around a dent or ding in it they replaced at some point who'll let it go for a reasonable price. If no one steps forward, I have a DG silencer with one of the mounting holes worn out (it would need some big fender washers to hold it on but it would work) laying up on the shelf I'd be willing to cut you a good deal on.
