Just the numbers 7,8 & 9 are not working on my keyboard

Broken? Possibly your "enthusiastic clean" may have left some type of residue or the alcohol may have seeped too far into the key switches and damaged one or more of them.

AFAIK, Apple does not repair their keyboards, so looks like you may have to spring for another one.

However, you can rule out your Mac itself being the issue by doing the following two things:

1. Reset the Mac's SMC (Intel only). Ref: Reset the SMC of your Mac - Apple Support
2. If you happen to have a spare keyboard (wired or wireless) try using it to see if you have full functionality with it.

Per chance did your keyboard you are using with your iMac experience a spill of any kind? If not, did you have a chance to closely examine the keyboard to see if any type of debris worked its way around those keys?

... and just to be sure, you are referring to an external wired or wireless keyboard ... correct? Would this be an Apple Wireless Keyboard or an Apple Magic Keyboard? If so, which exact model is it?