How to modernize this amp to work with microphone?

Is there a way to utilize this MS-2 as an independent microphone amp to sing along with a CD from another source, like a car stereo? Since soldering is not an issue for me, I would not hesitate to modify/add another electronic component inside this unit, to accept a separate microphone input. The microphone is needed to modify my voice. Another option would be to buy another portable amp, specifically design for a microphone, however, modernizing this one would be a very cool idea.