I work in motorsport – MotoGP – and the riders are subjected to random drug testing on an infrequent basis. I seem to remember that our riders, none of whom won a race, were tested twice during the season on average, immediately after a race.
It can be a bit boring sitting with a rider after a race in places like Mugello and Qatar (before it was a night race) watching then consume litres of water as they try desparately to fill the bottle!
To be honest, there wouldn't be much point in performance enhancing drugs in MotoGP, as it's not an endurance sport and the required level of fitness is fairly easy to achieve.
Whether banned substances are used for injury recovery in MotoGP, I don't really know. All I do know is that having had three very injury prone riders, none of them were given any suspicious substances to aid their recovery.
I've also never heard of stem cells being used to aid recovery of broken bones in MotoGP.