Under market rent in northeast Wisconsin

I'm in northeast Wisconsin and am looking to invest in my area. I found a property I like that needs enough work that I hope I can get for a good price. The issue I find in my area is the rents are very low which make it difficult to work the numbers. This property in particular has a tenant who has been there for 30 years and I don't think they raised his rent once. Although I would love to have a loyal tenant I'd have to immediately raise the rent by a significant amount. How do others deal with these situations? Thank you for your feedback.