How do you recommend converting this module for use with OSR games? (e.g. Scarlet Heroes, Labyrinth Lord, Old-School Essentials, etc.)
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Generally, I wouldn't. Maybe one of these days I'll make an OSR thing since I use those pretty regularly too, but I wouldn't convert the CGtG to work with them. NOW that said, if you just wanted to use it, I would point out that capacity damage is pretty analogous to regular damage in the original guide to growth with some balancing for the fact that in some cases exhausting capacity can instantly end a fight.
So, I would say start by looking at the values, costs, etc of various abilities or enchanted items. Then convert those to damage values smore appropriate for their level in OSR settings.