Convert a split ring induction motor to a squirrel cage?

Why not ?

Take out the rotor and replace it.

But then why do you want ? buy it, that will be cheaper.

Assuming you mean "slip ring" not "split ring", there is an issue to investigate first.

There are two kinds of "slip ring" motors, totally diferent from each other. A Synchronous motor uses slip rings for the rotor field connections, and a Wound Rotor Induction Motor uses slip rings for the rotor winding resistance connections. If you have a Synchronous motor, you cannot change that into an induction motor.

If you have a Wound Rotor Induction Motor, you can "convert" it, but it is rarely as simple as some people think. If all you do is short the rotor connections, the result is a Squirrel Cage motor with extremely high starting current and low starting torque; exactly the opposite of what most people need. It's better to leave it like it is and either use a Soft Starter to reduce the steps of resistance down to one, or use a very good VFD and short the rotor connections. If you don't know whaqt all this means, by all means stay clear of doing it.

Just check up the wiring principles (internal - that is windings etc and their properties and working principles, characteristics curves and why they are different ) of SRIM and SQIM and you will know why it should not.

It would be easier to replace the motor and keep the unmodified one as a serviceable spare.