Can anyone tell me if the black wire with a white mark from the motor is the positive wire or negative?
Never trust "colors" with DC, standards vary by country, manufacturer, and application :P. The positive is probably the marked one if its paint, but the motors used are reversible anyway. If the two motors leads get mixed(or inverted motor) it wont do damage, but will run in the "wrong" direction. Your chances of getting it right the first try by guessing are 50/50 :D. The direction of your locomotive in comparison to another loco, run on the same straight dc, facing the same direction will let you know if you have it 100% right. Or if you remember power pack direction switch and locos original direction, that would work too. I don't know how it would affect dcc ??? you might be able to reprogram your way out. ;D