I'd go a little larger to compensate for the loss of hp. 3hp is worth the extra at either end of the range. The drive pulley will need to be bored out to 1.125 from 1" if similar to mine. If the motor is not vector duty, your hz range is about 20 on the low end but between the two speed clutch and the back gear, you have some help. Torque and hp go down on the top end too and that is helped with the larger motor. English machines tended to have lower hp motors than they were built for. Probably due to lack of power being easily available.
I need a PM with email info to forward if you want to go that route. There are also threads on Practical Machinist that are worth looking at, including a vfd conversion that sounds like what you are doing.
Dave.