Rent-to-own + "sweat equity" contract

I'm looking at purchasing a commercial building that needs repair from fire damage, specifically a roof. The owner is willing to do a 24-mo RTO, with my labor + materials going toward the purchase price. Is there a standard rent-to-own + "sweat equity" contract that is used for such a situation? Of course I understand the specifics will vary.

Thanks for your input,

JBobo