No good fix for the carburetion, that's a 2 stroke thing since there's no power valve to broaden the powerband. The rear shock is probably not economically feasible to fix, but fresh oil and recharging the nitrogen will help. As far as front suspension and brakes go, finding a disc brake front end from a mid 90s YZ250 would probably be the best upgrade, and should be fairly easy to bolt up. Either that, or just try to ride within the limitations of the bike, and realize that with most riders, it's not going to be as fast or competent as more modern trail bikes. It'll still go 99 percent of the places they'll go, but it's gonna take more time and finesse to do it.