Transistor IRF520N not working as expected
I agree with JorenVaes that for most purposes (yours included) an IRF520N "is an IRF520N". HOWEVER, there is a slight possibility that the one you bought has different pin assignments than the original, so you might have it wired wrong.
On a datasheet, the values that you'll be looking at the most (for an N-channel MOSFET, like the IRF520Ns we're talking about here) are:


Id "Continuous Drain Current" -- how much current you can put through the transistor before things start melting or blowing up
Vds "drain-source voltage" how much voltage the transistor can effectively block. hitting it with more voltage than this will cause it to fail in one of 3 ways (let power through, but not enough to harm it {very rare}, melt {done it}, or explode {done it})
Rds(on) -- How much resistance the transistor causes when it's "turned on" ... this tells you how much voltage you're going to lose across the transistor per amount of current you ask it to pass, and how much power it's going to be dissipating (which heats it up)
Vgs(max) -- Same as Vds, except that it tells you how much voltage you can send to the gate pin (the one that controls the transistor)
Vgs(threshold) -- Tells you how much voltage you have to send to the gate before the transistor will start letting current flow through it.
Ptot "Power Dissipation" -- tells you how much total power (heat) the transistor can handle before it dies/melts/blows up (NOTE: this isn't how much power can "pass through" it, just how much power can be "lost" in it. i.e. it's the limit for current * voltage drop across the transistor.)