First, my memories of Gold Rush are rather vague. I do know for sure that the background picture shown is one of a bunch of similiar screens with a bunch of randomized trees on it. I think it’s when you are searching for gold, at the last part of the game. And the missing background from the opening isn’t shown there, since nothing was swapped, but rather two entries pointing to the same piece of code.
And the solution, well yeah, it’s a workaround, so it’s not perfect. I do not think the debug mode does strange things, since it pretty much only is a bunch of commands that get available. And about the variable, you can’t really know what it should be, but the “normal” speed setting uses 20, so I used that in the example. If you change the value to 1 the clock will be running like hell, 255 very slow. It’s indeed related to the computer you use, so just fill in a value and check how fast the clock is running (note that it is not running, so it seems, when a message is displayed on screen).
Furthermore, I suppose it happens on all systems. The whole script is simply missing from the game start, so I can’t see how it would work otherwise.
I think the MacNeill Bros. have nicely removed the copy-protection without any bugs, since they have, in contrary to crackers back then, the development tools and source code.