Both assembly and disassembly of this action requires control of the trigger spring. One of our members (Mike Priwer) devised a special tool to do just that.
The special tool consists of a piece of 1/4" or 3/8" wooden/plastic dowel rod cut to suit, to be installed between the compressed trigger spring and the frame interior opposite. Given that, the entire trigger assembly may be either removed or installed AS AN ASSEMBLY----the only game in town! The assembly consists of the trigger and both levers. The removal procedure goes like so: Remove the strain screw and hammer. Pull the trigger as far back as it goes (thereby compressing the trigger spring), and secure the trigger in that configuration (tie wrap). Install the special tool bearing against the top of the compressed trigger spring, and the interior of the frame opposite, and return the trigger to its forward position which will enable removal of the trigger ASSEMBLY (trigger and both levers). Note which levers go where, so you can install the assembly as an assembly. (It's at this point where my memory goes fuzzy---having done this only twice---a looooooong time ago!). The bottom line is you need to hold the trigger spring compressed with the trigger assembly, the trigger in its full aft position in order to remove the special tool---and both of my 1902's are long gone so I have nothing look at to refresh my memory. The action to accomplish this should be obvious, and if it isn't, send a PM to the inventor to bail you out----and to bail me out too!
Ralph Tremaine