The main difference between the two transistors are power and voltage ratings.
The 2N3055 is rated at 60 volts and 15 amps.
The 2N3771 is rated at 40 volts and 30 amps.
The 2N3771 transistor is rated for 40 volts which is low for use in your Kustom amp.
If you want to replace yours there are better rated transistors that you can use. Why do you need to replace yours?