if you simply want to change the ugly model but keep the titan and its abilitys then you need to get
Titan_PsiRebel.mesh from
"E:\Program Files (x86)\Steam\SteamApps\common\sins of a solar empire rebellion\Mesh"
use the convert too to convert to a text file then rename it to Titan_Psi_Loyalist.mesh
drop it in "C:\Users\{yourname}\Documents\my games\Ironclad Games\Sins of a Solar Empire Rebellion\Mods-Rebellion v1.02\{yourmodname}\mesh\"
you will probably have to move ability an weapon meshpoints to make the weapons look correct or you will have beams shooting from funny places (center of mass shooting) and your abilitys will look Weird!
check TITAN_PSILOYALIST.entity in "E:\Program Files (x86)\Steam\SteamApps\common\sins of a solar empire rebellion\GameInfo"
(will need to convert it to see the text)
you can also move the weapons from the Rebels titan Over that will probably be easier than working out meshpoints and directions on 5 weapons.
if you want to simply swap titans......
Open player PlayerPsiLoyalist "E:\Program Files (x86)\Steam\SteamApps\common\sins of a solar empire rebellion\GameInfo" scroll down to titan and change the name from TITAN_PSILOYALIST.entity to TITAN_PSIREBEL.entity
and I agree the Model is Dog ugly.. looks like an Urn....