The problem with the Engine Shot is the implementation. The effect of the ability is insanely good and could easily carry the Stuart into the late game were it not for the implementation being utter trash.
The solution? Change it into a skill-shot, like the Pershing's HVAP or the Firefly's Tulips.
Give it a standard aim time, 150 penetration at Range 0 to 10, then have that rapidly drop to 0 penetration at Range 20.
150 penetration means it'll penetrate the rear armour of anything (it's meant to be an engine shot!), but it can bounce if you fire it at the front of mediums up.
You've now turned a super-janky ability into one with a lot of tactical depth.
YES YES YES. Stuart is almost NON-EXISTENCE in 4 v 4 game mode because of how clunky this ability is. I would def build if its re-worked.