Can I use a NPN Transistor as a Varactor Diode?
Yes, you can use the BE or CE junction of an NPN (or PNP) transistor as a varactor diode, however it may not have the capacitance or capacitance range you want. The BE junction may not have the breakdown voltage required. 

The NTE613 has a 22pF capacitance at -4V and a 2.9:1 range from -2V to -30V. That seems on the low side to start with for an AM-band radio- I would expect several hundred pF. Perhaps it is not operating in the usual ~530 to ~1600kHz MW band. 

Anyway, a couple 1N4004's in parallel will be fairly close to the NTE part. 



As you can see, the higher voltage parts use a different (PIN) construction so they have less capacitance.