nice job on the rebuild, as far as a bumper i think this one would look pretty sick on your bike BLACK DG V-PRO FAT SERIES FRONT BUMPER YAMAHA BLASTER : eBay Motors (item 290401067985 end time Apr-10-10 15:44:28 PDT)
the backfiring problem sounds like your timing is off take off the left side cover and remove the flywheel (youll prolly need a flywheel puller) and check to make sure the key is on the crank