What ROM, S-On or S-Off?
You need to get the market installed first. So a flashable zip, installed from recovery would be a solution if S-On. Don't have one to hand, but there are various market zips at XDA. If S-Off could try uninstalling, cleaning dalvik, then manually installing an apk. Either way a web search should dig one up.
I'd normally recommend a gapps package from goo.im, but it's not so easy to tell the market version there - you'll probably get something newer.
You can also download a ROM that comes with the old market (e.g. dGB) and extract the apk from there. Even replace the one in your current rom zip with that and reflash, if you are happy editing zips.
You always take a nandroid before flashing anything, of course