Any REASONABLY sized motor witll RUN the machine axis when you DIY retrofit them - thus, a SWAG motor selection could end up to be quite adequate.
HOWEVER, when it comes to fast rapids and abruptly quick accelerations, you need to consider how fast you ultimately want to move it in order to properly engineer the motor sizing and/or the gearing needed to connect them.
A more scientific way to SWAG the motor size is to match the motor sizing already being used on a machine with comparable performance that you're trying to achieve.
As far as Bridgeports go, the classic mill was fitted with fairly healthy servo motors when they did CNC variants that became Eztraks and/or V2XT's - I have one of each but don't know the motor sizes off hand. Can't speak for the stepper variants used on the machines that preceded them.
The following thread may also be of assisance to approximate motor size calculations:
http://www.cnczone.com/forums/showth...078#post214078
Specifically post #15