Is it a trans fluid leak or a bevel gear leak? I have replaced many collars and resealed many bevel gears. If its just the collar seals then replacement isn't required unless u see visual wear. It can be slid out after removing the bevel gear and then seals popped out and in. Sometimes however the collars can be seized in there in which removal can damage the splines.
Figure removal of the bevel gear would be a few hours and collar replacement would add another hour to the total repair.