First I'd like to point out that the 1-508th was assigned to the 193rd Infantry Brigade in Panama till '97 irl.
As far as a light motorized battalion, something you might want to consider using is a TLAT battalion.
http://www.orbat.com/site/history/op...talion1980.pdf May not be exactly what your looking for but they are light and motorized and it would kill two birds with one stone a reserve unit and motorized. Another option to fill that hole is to say the NG reactivated the 1-143rd Airborne again in TX. As far as which battalion is motorized, etc. thats pretty much up to your discretion. As far as my suggestions for the unit designations I'm more of a historical guy so I would take one of two options here. A. Go with 2-, 3-, 4-503rd and one of the NG options plus I'd change the Cavalry Squadron (I'm assuming you're using a whole cavalry squadron by your designation and not just a troop.) to either the 4-16th Cavalry or the 5-17th Cavalry Squadron which actually had historical ties to the Herd from Vietnam (via their lineage through D Co 16th Armor and E Troop, 17th Cavalry respectively). If your using just a troop then historically it would be D Troop 16th Cavalry or E Troop 17th Cavalry or if you want the latest flavor A Troop 91st Cavalry. now on to B. Keep the 1-503rd and say the one in Korea got reflagged to another unit designation like the 2-506th or something at some point previous. Thats my tastes your tastes are probably different and I'm curious to see how you work it out.
As far as the 228th Aviation Battalion, when the 1st Cavalry began converting to Division 86 configuration. The division added a second aviation battalion to go along with the 227th around 1983. So they went with the 228th for it which makes sense given the history of the 228th. That designation stuck until they regimentalized the aviation branch in the mid to late 80s eliminating the separate aviation battalions designations. So if your going with the early 80s designations which is what is used in Twilight 2000 and by Frank then 228th was already assigned prewar to the 1st Cav.