I know this is a generalization that skips over a lot of nuance, but in real games you should trash it more often than not. It’s usually worth it to trash it and buy another vs keeping it, unless you have enough money for the card you would be gaining in-hand and would just buy another Engineer anyway.
If you want Engineer in your deck, but don’t have enough for one of your gains, trash it, buy another Engineer, carry a little debt, no big deal. If you don’t want Engineer in your deck anymore, of course you trash it, duh. So this covers most early game Engineer situations.
It’s generally better to carry a little debt in exchange for good cards in your deck sooner!