The kingpins from 870C through YZ10 are basically same shoulder screws. even you can find the replacements from Tamiya and Kyosho as well. and MX4 kingpins are flanged bushing (flanged sleeve) which are easy to assemble and hard to be lost
The flanged sleeves should be assembled from inside to out on the front C hubs.
But, YZ10 C hub has a bit small room for MX4 kingpins. and both assembled from top to bottom. like these pics
All done with titanium screws, and ready to run.