HTC Desire bravo custom rom
While doing similar research to upgrade an old HTC Desire I found this article that may help you: Cant add Google account to HTC Desire

It points out that you first need to upgrade Android Market to Google Play.  However in other forums I read that you have to have Android 3+ to do so, though I'm not certain that's true.  But hope that leads you in the right direction.

Also, if you already figured out and answer, please share back, I'd love to know (it may save me time).  Thanks!