If you haven't opened all the krypts, check the TRMK guide to find which coffins have koins.
In game options, you can change the settings for how often the "test" mini-games appear. Set it to minimum to speed things up. You could also set the CPU to Easy to get to the mini games faster.
There is also a bug which lets you accumulate koins by wagering. I don't know any details on that though. Search TRMK or Google.