How to choose equivalent transistors for replacement
The most important parameters depend on the application, but the main ones generally are:-


Type (Silicon/Germanium, NPN/PNP, Darlington, Audio/Power/RF etc.)  
VCEO (Collector-Emitter breakdown voltage) equal or higher than original.
IC (maximum Collector current) equal or higher.
HFE (DC current gain) equal or higher - but not too high.
Case/Package. (Will it fit? Can it be kept cool?) 
PT (maximum power dissipation). This rating is often 'optimistic' (actual power handling is mostly determined by the case and heat sinking).


Some circuits are critically dependent on other parameters such as VCE(sat), safe operating area, capacitances etc., while others were designed around popular 'industry standard' components that may not be optimal (and might even work better with more modern parts). Often you can get away with using a generic replacement, but you should always monitor its performance in the circuit to make sure it is doing the job properly. 

The MJ900 is described as a PNP Power Darlington in TO-3 case rated for 60V, 8A and 90W max. To match its characteristics You will need another PNP darlington transistor, in a case that can dissipate similar power on the same heatsink. If your circuit runs on low voltage and current then these parameters could be relaxed. Darlington HFE is usually quite variable so the circuit probably isn't very sensitive to it, but avoid transistors with built-in Base resistors since their gain drops off at low current.