I have no direct experience, but I do recall reading an article in a magazine, probably Muzzleloader, in which they tried one of the L&R RPL series locks. Not sure if it was the one for the Lyman GPR or not.
Anyway, it was not just a simple drop in, but it required some wood removal. Nothing to get excited about.
Quality (according to the article) was typical L&R, and better than the original lock.
I can try to find the article and get more details for you, but I am vacationing in Colorado with my wife and won't be home until next Wednesday. Sorry.
Dale H