try a new spark plug first. thats the easiest and cheapest repair. 90% of all small engine problems are fuel related. so i would start at the top and go thru the entire fuel system thoroughly. the gas cap has a vent. is it clear? is the tank clean and rust free? is there good fuel flow from the petcock (shut off valve)? is the filter on the petcock clean? is the fuel line in good shape and clean? is the carb clean inside?
all these questions cover the entire fuel system
but you may have another problem elsewhere. you said its hard to start so, this again leads me to believe you have a fuel issue, but if everything i asked is clean and you have good fuel flow, then you need to check compression with a gauge.you want to see 120 lbs or higher. anything lower indicates a problem someplace, either worn rings or bad seal.
but stalling as you slow down orstop may be a bad crank shaft seal, or sticking clutch which does not disengage and allow the engine to run freely as the bike stops. go thru the fuel system first and do a compression test. once that is complete we can figure out whats wrong if the problem persists.