If you're trying to decide whether to sell the property or continue selling it, I don't think ROI is the best measure. ROI would tell you what your return on your initial investment is, but since the past is past, you can eliminate "sunk" costs and investments, and just look forward to make the best decision.
A present value analysis would tell you which is the "best" investment from a pure finance point of view (not taking into consideration stress, hassle, enjoyment, etc.)
You have described two paths:
- Sell the house. That's easy since you can probably ignore time-value of money and your "present value" would be the expected net income (sales price less closing costs, commissions, expenses to get the property "sell-ready", etc.)
- Pay to fix up the property and continue renting. This one isn't too bad either since you could calculate the present value of the interest payments (which should be negative). You can use just the interest payments since you should be able to assume that the principal adds to the value of the property, which you'd recoup when you sold it. Then, calculate the present value of the rents as a perpetuity (either growing or not growing) using a simple formula. An appropriate discount rate would be whatever rate of return you would expect on whatever you would invest the proceeds of the sale in something else (10%? 15%?).
Whichever has the highest present value is the "better" financial choice.