You don't need a lot, just something lined up on the correct axis with enough engines and fuel to slow the station down enough to fall out of orbit. The station has its own propulsion (on Zvezda & Zarya, I believe) even with most of its station keeping being down by Progress etc. There's no reason they couldn't loft something with an F9 (23t to LEO) or maybe FH (64t to LEO). With the fact that it's going to burn up in atmo anyways, you can strip a lot out of the existing cargo dragon.

You can slowly deorbit anything with enough time/fuel. It's just a matter of how fast NASA wants to make it happen to assure it lands in a specific spot and doesn't breakup too much while still in "orbit" (see also: atmo + solar panels).

According to https://forum.nasaspaceflight.com/index.php?action=dlattach;... (PDF)

> The deorbit vehicle will need to provide at least 3236 N thrust to hit the target delta-v within a 60-minute time period.

Each Draco on Dragon can deliver 400N, there are 16, that's 6400N. So unless I've missed some math somewhere, a Dragon could do the deorbit. Ignoring the fact that they probably haven't burned those engines for an hour. Also fuel required for that manuver.

Modified Dragon seems like the simplest option. Dock and then burn retrogade while attached. Probably not nearly that simple in reality though.