If you got Mach3 from "reputable Chinese supplier" (LOL), just remove it from your computer (keep plugins, if any, as well as machine configuration), download .62 (most recent version of Mach3), despite that is, I guess, more than seven years old, reinstall it, buy yourself a license, copy back your machine configuration, plugins, ... and you're good to go without 500 line limit.
(waiting for Joe to say that you should buy Mach4

)