I think an engine change is a must.
You can still make it as moddable as you want, the important thing though is having the bare minimum of nice looking graphics (even if they're cartoony) and fluid animations and sounds.
The main reason i can't enjoy soldak games for longer than an hour is the lack of fluidity. Most spells feel bad to use and overall gameplay is annoying.
Gameplay for me was basically
* step on trap, ouch sounds
* invisible aliens start appearing around me
* keep chugging potions and attacking
* stones start falling from the cave roof
* i step on another trap
* trap spills liquid everywhere, multiplying
* my aoe spell makes the game lag to hell because it hits all the tiny liquid droplets
* more stones start falling
* i die
The core gameplay loop really must be better than this. Even if graphics are good, i still wouldn't play more than a few hours.