for bolts, have a look at the picture with the color rings; all M3 + M3 locknuts: green; original motor bolts from your UM2 pink; M3 x 30 (longer is no problem) yellow; m3 x 25mm, longer is no problem (small head, needs to be able to sink into the bearing arm. The bolt holding the spring is a m3 x 50mm (minimal) with a hex shaped head. If you can't find one with a hex shape make one by taking another bolt and put a hex nut on it against the head, fix with loctite. Use a locknut to hold the spring (inside the printed knob), to avoid unwanted change of pressure. the Blue bolts are M4 x 20 (or longer) I used butterfly nuts, but there's no real need for it. The spring and bearing are from the original UM2 feeder.