Suggestions for matching bad sound?

I have strange question that I would like some help with. I have an old cassette recording of the band I was the singer for in high school. It is of a performance we did at our school in which an unbelievable number of things went wrong. I have always wanted to enhance it, especially the vocals as the harmonies on "Sweet Emotion" are tremendously bad. (The rest of the band didn't believe in rehearsing vocal harmonies) Fortunately, after the first song the other guys quit trying to harmonize making my work on the other songs a little easier. In fact, my avatar is from this very performance.
My question is what suggestions can anyone offer to lower the sound quality of my new overdubs to better match a 27 year old cassette recording done on a regular stereo cassette using 2 mics. I have toyed with the idea of cutting some EQ frequencies and running it through an amp or amp simulator, but does anyone have any other suggestions? I am adding vocals, percussion and keyboard parts, especially keyboard parts to try and reinforce the bad harmonies. (I know this will not fix these bad vocals since I can't remove them, but so far I have been able to make them sound a little better)

Ooooh, what a nice idea! I hadn't thought of trying that. Thanks Rev!!!!