Can I vent plumbing in an old house to the crawl space?
If allowed in your location an air admittance valve can be added. It needs to be close to the level of the drain. I put them in the sink cabinet as close to the top as I can. I agree with @kris my 1930 farm house has the vent outside several Victorians I remodeled also had them outside. But an AAV if allowed is the easiest but not in the crawl space.