Firstly, well-done for completing a change of the HD for an SSD. These are tricky little devils to work on (especially the first time round). Did you use the 'quick and dirty' method, or the full-monty approach - removing the screen and then the EMI shield complete?
Like AphoticD, I'm inclined to think that you have the keyboard connector cable not fully seated. As you now know, access is easy enough to the cable, but rather than giving it a harder push to fully seat it, I suggest pulling up on the plastic loop to disengage it completely. Then (as already suggested) give both ends a clean with isopropyl alcohol, then with an air duster spray can, ensure that there is nothing hiding between the pins. Give it several good blasts of air. Refit using gentle but adequate force to ensure that the connector is completely seated.
I'd be surprised if this didn't cure it.
Taking the worst scenario, if that didn't fix it you're then obviously into looking for a replacement keyboard. In my experience they are hard to source, and the alternative would be to find another cheap clamshell, as a doner unit. Here on mainland Europe they can be found from 60euros (70 USD) fully functional, albeit with AZERTY keyboards. Laptops here (France) with QWERTY keyboards are like unicorn headgear.
Pls let us know if a clean & reseat of the connector cures the problem.
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I've since seen that there are Clamshell keyboards available from a Mac spares specialist in the UK, 'The Bookyard'. These would of course be QWERTY keyboards.