Two-family investment property: Convert one unit to natural gas or replace oil tank?

Hi all. I own a 2-family investment property in CT. The house is 100 years old; the heat is oil. Each unit has its own oil tank and boiler in the basement; both vent into one chimney. I just learned that one of the tanks has a small leak, so I'm trying to decide how to move forward.

For context: The house is located in a desirable area, and my tenants are all high-income earners, so I'm not worried about their ability to afford heat. They pay for heat, and I require them to have a contract with the oil company so tanks are automatically refilled as needed. I pay for the annual maintenance cleaning, which is about $500 for both tanks/boilers. While I've never had the place empty, I have noticed that qualified tenants have sometimes declined to submit an application once they learn the heating is oil. So I feel like if I make this investment, I may increase resale value (though I have no plans to sell at present), and it may make the property more desirable to high-income tenants moving forward.

My options, as I understand them (these pertain to the one rental unit only; the other is functioning and doesn't need altering at present):

1. Replace the estimated 75-year-old Granby tank with another stainless steel 220-gallon Granby tank. Price: $4,500
2. Replace the Granby tank with a smaller 160-gallon Roth (larger ones won't fit down the stairs, given a turn). Price: $4,000
3. Convert to natural gas. I've confirmed that the house has viable pipes, capped, locked, and ready to go. I've been told I can install a high-efficiency wall-hung gas boiler that vents outside. Price: $10,000
4. Convert to natural gas by installing a gas conversion burner to the existing Crown boiler. Price: $3,500, plus an estimated $2,000 for a chimney liner. I'm not clear about this last bit, as I can't imagine the other oil tank/burner for the other unit can vent into the same chimney as natural gas. I have this question into the contractor who quoted it; perhaps I misunderstood something.

All prices include disposal of the leaking tank and its contents.

Thoughts? Recommendations? Not yet sure if there are rebates/incentives/financing options on the natural gas options; need to inquire.

Thanks in advance to all of you who weigh in. I'm just learning about this part of owning a home, so it's new and unfamiliar.