hey welcome to the forum
the engine may be ok.. it could just be the starter that is jammed.
them 903cc engines are pretty indestructible.
if you want to do an engine swap you could use one from out of a cinquecento.. the 899cc OHV engine is the same engine as the pandas original 903cc engine, so all the engine mounts and gearbox will line up perfect. its also injected with an ECU. if you get the early 6F ECU it only requires 3 wires to connect it to the car. grab a 2 bar in-line fuel pump and the engine swap is complete.
the fire engine swap requires the fire engine, engine mounts, the gearbox, gear box linkage, the drive shafts, coolant system and throttle cable.
you will have to drill holes in the chassis and weld nuts in to hold the engine mounts in place.
personally i would go for the 899/cinq engine conversion. fuel injection on a panda and the ability to start the car first turn of the key in all weather out ways the carbd fire engine.
in terms of power, the 899 engine is 43hp and the 999cc fire engine is 45hp. however if you got the fire engine from a panda 4x4 you could get 50hp. (its the cam shaft that gives the 999cc fire the 5hp boost in the 4x4 engine)
hope this helps you out