The front cover is a tricky beast to seal. Getting part 143 (horseshoe gasket) in place properly is most but not all of the battle. The rest is using and applying the proper sealant. Check the archives.
Part 142 is just a seal but you might as well replace it when replacing part 143. In fact I'm from the camp that says to put part 143 in place then fit part 142, as opposed to the manual that says to fit part 142 into the front cover first. The front cover has to come in at an angle to keep part 143 in place so there's a chance to nick part 142. At least that the easy way to get the front cover in place.
I think part 139 is noted as a replacement item because if it takes a set on the cam, you can damage it when removing it.