Can anyone recommend a cockpit replacement that works with the Revell MIG 21PF 1/48 kit?

Are we speaking of the old beast form the 1970s? I don't think that anything was specifically made for it so you'd need to adapt a set made for another kit. The Pavla set for the Academy kit might be able to be adapted.

Outside of the FM set for the Academy MiG-21PF, there have been very few 1:48 MiG-21 update sets that are for early MiG-21 variants - most are for MF/SM or later variants.

With Eduard's MiG-21PF series starting later this summer, I expect we will see the FULL Eduard treatment for the MiG-21P family (Brassin, Photoetch, etc.) but, obviously those will be for the Eduard kits.

It maybe possible to modify those for the Revell kit, but keep in mind, the Revell is inaccurate, and consequently, will have dimensions different from the Eduard kit.