Seeing as it's a new blaster (to you) I suggest doing a compression and leak down test. List any mods, pipe, air filter, box lid (Y/N?)
Either find the PDF manual on here, or get a Clymers repair manual.
Go over the machine with a fine tooth comb, write down all issuies and fix in order of importance, brakes being #1
As to the running, start with NGK B/BR8es plug, don't understand "too small" of plug, don't matter, get right plug.