I am not sure whether the concept of nonprofit capacity is strongly related to that of nonprofit sustainability. But if so, my recent paper dealing with nonprofit sustainability might prove useful: https://www.researchgate.net/publication/280315948_Sustainability_of_Rural_Nonprofit_Organizations_Czech_Republic_and_Beyond?ev=prf_pub
My approach would be to conceptualize nonprofit capacity as the intersection of what economists call "demand-side" and "supply-side" determinants. Does that make sense?
Article Sustainability of Rural Nonprofit Organizations: Czech Repub...