Only the 51213 TA05 center pulley fits the TRF501X.
http://www.racingfactory.fi/shop2/ca...ils.php?p=1186
The other alternative coming soon is the 53989 Center Pulley (18T), should be available in June.
It remains to be seen if Tamiya will offer a centre one-way option in the future, but that would require a new drive hub and one-way pulley.
For now the only one-way alternative is the 53951 TRF501X Front One-Way.
http://www.racingfactory.fi/shop2/ca...ails.php?p=855