The MP Select Mini uses a Right Hand MK8 Extruder. A few acceptable replacements are listed below. Some listings will have a choice of left or right hand model. Be sure to choose “right” or “right hand”.
Watch Thomas Sanladerer calibrate his extruder.
GigDigit - Extruder Gear
Amazon.com - Anycubic All Metal Right Hand MK8 Extruder (Gold Color)
NOTE: Requires two M3x20mm screws or two M3x25mm screws and maybe a couple of M3 washers that are not included.
NOTE: This is optional but if you would like to keep the original/stock extruder gear, you will want to also purchase a 4x10x4mm bearing (examples linked below). The benefit is that you won't have to struggle to remove the existing gear and you won't have to recalibrate the extruder. Removing the existing gear can be troublesome but applying heat to the gear with a heat gun, lighter, small torch, or similar should make the task a little easier due to the thermal expansion of the gear. This should go without saying but please use a tool or something other than your hands to grasp the gear after heating.