Hi there! Sorry I've taken so long to respond to you, I expect that you've already house ruled something of your own in by now? The way casting works with FORGE is really more like a mana system than spell slots (in the 5e sense anyway, with individual slots per level). E.g. a 6th level caster with 6 'casts per day' would be able to cast the same spell 6 times if they wanted to, before being spent until they rest.
I did initially try to implement a more involved mana system, with higher circle/level spells having more of a cost, but it became a little bit convoluted and straying from the simplistic design goals.
Roll to cast should fit in fairly easily. I would roll d20 +relevant attribute check bonus against 10+ the spell level to cast successfully. Nothing to stop you incorporating the crit results straight from shadowdark either, if you like those, or you could roll a random event where it is automatically assigned a positive or negative spin, depending on whether it was a nat 20 or 1!
Let me know how you got on! :)