The RPG aspect is trivial to recreate in Unity honestly. It would only take a few hours of work to get that end down, then maybe a week or two of work to get all the skills and passives back in and working. Hell honestly anyone with some willpower and the ability to search youtube could probably get this done.
Its the way the combat is setup and actually played out that is the problem. The number of states required is particularly high, the amount of tracking is high, and there are tons of modifiers for each state as well... and the more states you have the more complicated their interactions become.
Its not impossible to do by any means, or even really difficult, but setting it up and testing it would literally be a full time job. Most modders do this stuff for their own enjoyment, so finding one willing to make that kind of time commitment is extremely rare. If you add in the skill requirements (tutorials on youtube are not enough for this level of coding, you would need some actual skill) of actually creating something like this on top of the time investment requirements... Well lets just say the odds of someone stepping up are abysmal. Although I'd certainly love to see it happen.
As far as the art assets end, honestly its hard for me to speculate on. Its so far removed from my area of expertise that Im probably going to embarrass myself speaking about it. But considering nearly every item is displayed both on the paperdoll and the players sprite, Id imagine your probably not going to be able to get away with free shop assets.