I purchased a generator transfer kit that came with a 4 prong 30 amp cord that connects the generator to the transfer kit. The problem is that the outlet on my generator is only 20 amps. Can I remove the 30 amp plug and install a 20 amp plug leaving the 30 amp plug at the end that plugs into the transfer switch?

Yes, however, you will be limited to the 20 amp output of the generator.