Is it possible to dismantle and refurbish an original 870 Technic motor? I have a couple of ancient 870 motors (the 4.5v ones) that run but don't sound great. I've looked for, but can't find, a guide on how to dismantle them. I'd like to either get in there and clean/lubricate them selectively, or perhaps even replace the motor element.

The end cap where power is supplied is clearly where they went together, but some gentle wiggling and pulling suggests there's probably some internal one-time assembly clips.

Is there a trick to get them apart?

Edited to add:

Further wiggling and poking makes me believe that unfortunately any clip is in the curved sides of the motor casing, which are very resistant to flexing. A thin blade passes into the top or bottom straight edges with little to no resistance.
While it is important to keep old and rare Lego pieces in good condition;
looking at the part in LDCAD, it does not look like there are any screws to take it to pieces, so I think it is internally clipped together, where such clips cannot be pulled back apart again after they snap in. Several Lego motors use these clips.

However since you cannot see them it is debatable weather it matters if they are broken/cut because you can probably super glue it back together and since the glue will be on the inside you won't see that either, but I will leave that risk up to you :)