I don't have a 300 but I suspect that you could do a disc brake conversion (may require some fabrication to work?) if you wanted too badly enough. You can put used brake drums on the bike too, if new ones are unavailable. Are your front brake shoes in good condition and the brake adjusters screwed out where they belong? One or more of those may need to be readjusted...? Another possibility is some air needs to be bled out of the front brakes hydraulic lines/wheel cylinders.