The Monoprice Select Mini is an inexpensive solid little printer. I have used mine for many hours and made lots of custom improvements to it. At some point the the controller board failed. I contacted Maylan the original manufacturer of the board for a replacement and was quoted at $66 for a replacement ($45 board, $16 shipping, $5 currency conversion). Rather than buy another board that might fail for a third of the cost of the printer I started researching my options.
Many of the kit 3D printers use Marlin on the Arduino Mega and a Ramps 1.4 board to control them. I decided to buy the parts and see if I could set Marlin up to talk to the Monoprice's existing hardware. Marlin is an open source firmware that is easily compiled and deployed to an Arduino compatible board. The configuration in Marlin is well documented and easy to update. You can find many people selling the parts you need in a single kit in the $30 - $40 dollar range on Amazon (http://amzn.to/2kXxSRq) or Ebay and if you part out the individual pieces you need on places like aliexpress.com you can often get it for even less but you will wait since most of these will be shipped to you via China Post.
These are the basic parts you need to do this project: