A card, like tank factory, you can place adjacent to a unit, that repairs any mechanical unit something like 20-25hp each turn the unit stays right on the workshop (or make something like a repair hex next to the building itself).
Cost could be around 100g.
Possible things to add:
- unit only gets repaired when it doesnt move at all during the turn
- unit can't shoot while being repaired
- enemy units will be repaired too
- units on the repair hex get an armor decrease

and the repair-building can only be attackt if there is no other unit adjedent to this building
That means your units first attacks the enemy units then the building