Can landlord provide a countertop oven instead?

[Suduk N.]

The large gas oven is broken at my rental. The replacement cost is very high because it is an old small size wall oven and not many manufacturers make that size anymore. 

Can I provide a countertop oven instead lawfully?

Or Am I legally obligated to have the large oven appliance working?

[JD Martin]

You can do anything you want until/unless you have to answer for it in court. Then the question would be 1. Did you provide an appropriate substitute, and 2. Is the substitute materially equal to the value the tenant rented the property for in the first place? Considering that it's unlikely anyone would sue you to find out the answer from a judge, you can probably just provide the second option.

The bigger question: why would you want to? A rental unit that's not some million-dollar jobber should always use standardized appliances so you can swap them out if/when they break. Since this unit apparently wasn't rehabbed, now you have to ask this question. If it were my place, and I didn't want to rehab the kitchen, I would have the existing oven repaired or replaced. It's not your tenant's problem that the cost of a new oven that's 50 or 60 years old is very high :)