Will a usdm ecu work on a jdm motor?

Yes. Depending on the motor, the ECU may give you a Check Engine Light. For instance, JDM motor's do no have evap, but USDM ECU will look for one. When it doesn't find one, it will through the CEL on, though it won't affect performance.

However, if the motor is a JDM B18B (possibly other models as well), you will have to do a CKF sensor bypass. It's fairly simple, and is mandatory in order for the car to run well. Basically the JDM motor does not have a crank sensor, but the USDM ECU will look for one. This will affect performance because the ECU will throw a faulty/defective/missing CKF sensor code, and through it into limp mode to protect the motor from damage.

To bypass this, you need to do some simple wiring. This can be found here:

http://www.ff-squad.com/technet/ckftrick.htm