Converting a 120V universal motor to run on 240V
You do not tamper with the motor. You buy a step-down transformer rated for the current the motor needs.  Get the voltage and current ratings of the motor, then just search the web for 'step-down' transformers that match your 230 to 240 vac line voltage on the primary side, and 120vac on the secondary side.The transformer needs to handle the start-up current as well (it may be listed on the motor), so it maybe rated for twice the motor run current - or more. They are plentiful and have many sources and should not cost much. They are a quick and safe solution.