As I promised, I installed the fixed button.
MPC-2000XL is about to be opened:

I decided to replace "Stop" button, because I use it more than tap tempo. As you can see, the button is repaired with tape. It works, but it feels too loose. For tap tempo and "0", brand new buttons have been installed already.

Replaced:

Close up photo:

The replaced button. It is repaired with tape from P-Touch:

Off the topic, my pad is sticky and there are too much gap between sensor and pad, so I've been using this electrical tape trick. It works very well. I have to re-apply the electrical tape occasionally though.
(I was changing the tape, tape is applied to all 16 pads.)

The MOLEX connector was very loose when I got it from ebay seller. Because of the loose connection, "Full Level" and "16 level" buttons sometimes wouldn't respond. I tried many ways to secure the connection, and by inserting cutout post-it paper underneath the MOLEX connector worked the best. (I did not want to use glue, as I wanted to be able to disconnect again.)

The button works great so far. No more loose feel. So far, it feels identical to brand new button.