If you are currently running at a sprint track, in Formula 125 (2004 IKF National Class #7), the most likely road race crossover classes are...
 
 

IKF National Class #15 - Formula 125 Limited

 
Rule Differences...
 

Tires:
 

No compound or manufacturer restrictions.  5" or 6" rim diameter.
 

Intake:
 

Air filter may be used, in place of an air box, unless local authority mandates differently.
 

Exhaust:
 

Exhaust silencer body minimum 2.5" diameter and 11.5" length.
 

Race / Heats:
 

20 minute heats.
 

IKF National Class #17 - Formula 125

 
Rule Differences...
 

Tires:
 

No compound or manufacturer restrictions.  5" or 6" rim diameter.
 

Intake:
 

Air filter may be used, in place of an air box, unless local authority mandates differently.
 

Exhaust:
 

Exhaust silencer body minimum 2.5" diameter and 11.5" length.
 

Race / Heats:
 

20 minute heats.
 

NCK Local Option Class #29 - West Coast 125cc Sprint

 
Rule Differences...
 

Tires:
 

No compound or manufacturer restrictions.  5" or 6" rim diameter.
 

Intake:
 

Air filter may be used, in place of an air box, unless local authority mandates differently.
 

Exhaust:
 

Exhaust silencer body minimum 2.5" diameter and 11.5" length.
 

Race / Heats:
 

30 minute race.