Change it in the Aircraft's data.ini file (F-19 DATA.ini for example); I'm on my laptop right now so don't have access to the files/lines, but there should be a NATION or NATIONNAME or something. Switch it from USAF/USN/USMC/whatever to SOVIET. Though, if you want to keep the plane on the other side too, make a copy and rename the enemy plane. When you do, you'll have to change the plane's base ini (F-19.ini) to what you named the plane (say F-19SOV would be the folder name, so the plane's .ini would have to be F-19SOV.ini; the plane's data.ini shouldn't have to change, but the NATION line still has to read SOVIET in the data.ini for it to be on the Red side.)
I did something like that to make a TOPGUN install where the red force is comprised of F-14, F-18, A-4, and F-5 types, while the blue force has the same planes and also some AF types (F-15, F-16) as well.
EDIT: Also, for weapons, using the weapon editor you can add a nation to the weapon; so if you checked off "SOVIET" for, say the AIM-54, you should see it available for Soviet A/C with big enough AHM pylons, or add USAF or USN or NATO (etc) to the weapon pylon you want to carry a Western missile on an Eastern plane.