When I originally saw this question, my first impression was that this might be a challenge. The Courteney has a light round barrel and a lot of aftermarket barrels I've seen are a heavier octagon in larger calibers than 30-30. Also going down in size requires the rebarrel instead of a rebore of the existing barrel. So my first question would be "Do you have a compatible 30-30 barrel available or is that part of the problem too?".
If I was going to attempt this, I think my personal solution (which you might hate) would be to sell the Courteney and use the money to buy a custom C Sharp Arms 1885 in 30-30 Winchester. From a financial standpoint, how much more money will that cost & convenience will that save compared to buying a new barrel and hiring a gunsmith to rebarrel the current rifle? https://www.csharpsarms.com/product/custom-1885-highwall/
Plan B might be to contact C Sharp Arms Custom Shop & ask them if they would put one of their 30-30 Winchester 1885 barrels on the Courteney. They may say "No" but if they'll sell you just the barrel then any competent gunsmith might be able to finish the project.
As you can tell, this is all speculation on my part & I'm not bringing any practical expertise to the topic.