If its a dead motor, it is a "standard" can size, so there are lots of drop-in replacements. With 30 years of progress, probably a lot of better ones.
If it is gears, replacement gears are available for users in other branches of radio control like planes and cars.
Replacement prop shafts and props are also available. Graupner didn't usually go for unusual shaft diameters, so there is a wide range of possible replacements, just a question of knowing which bits you want.
If you choose the brushless route, you will need a new speed control as well – with brushless, the motor is only half of the deal, you absolutely do need a speed control to work it. And a servo tester to try it without a radio. And probably a setup card for the ESC.