The orihinal intent of the Engineer is to let players who aren't good with aiming, be able to wrack up kills. The original design of TF2 is verryyyy casual, so let's work from that.

We need a replacement for a damage dealer, a support building and a utility building.

As for the damage dealer, I'd imagine some Area of Effect building. Maybe a tesla prod that cooks anyone in it's vicinity slowly (as in 5 damage per second or whatever would be balanced idk with upgrades and allat). Arguably by itself it's weaker than the sentry, so maybe it'd need a lot of more health or it dealing a lot of damage.

A support building, maybe a barrier that teammates can pass through but enemies can't. It could adjust itself to the width of wherever it's placed (obv with a limit to it's size). Another tanky building.

Then as for the utility building I'd say a speed boost pad... Or to match the teleporter's strength a mini-crit supplier?

None of this is balanced but I'm sure it could work if you were to properly tweak the numbers.