Are there plumbers who are willing to replace galvanized drain pipes in crawl spaces that are barely accessible ? And if so how much per hour?

Any plumber can do it. Go on line, Google "Service Plumber in your area " and call them.

Replacing pipes under house is a dirty work. If sewage was emptying into the crawls space for longer period of time than the sewage needs to be pumped out and dirt treated to eliminate the smell.

Deal with it in timely manner as such leaks are health hazard.

Usually, such work is not charged by the hour, but by the job. Get three free estimates and select the plumber ( or price ) you like the most.

Milo

Agreed, I would assume that any plumber does it. I would be shocked if there are some that don't. That is their job. And yes, not by the hour, but by the job,

Not "ANY PLUMBER"...