CoolhandMatt,
Use the search function with words Divorce and Mp5k or SP89 and I bet you'll get hits galore. Many have found themselves in this situation including myself. My SP89 has a Fleming sear in it and is papered just like yours I think.
Here are you approx choices ( as I see them, your mileage may vary, yada yada)...
1. Just keep it as is. When you want to use that pack on another host...just make sure you remove the foregrip and stock from the SP89 first ( "sell" it to a nearby friend or whatever...just avoid the nebulous "constructive possession " bit). Dont bother SBR'ing or writing letters or anything. This avoids the unregistered SBR issue altogether as its now a stinking pistol only.
2. SBR the MP5k using a form 1. You'll have to engrave the current guns owner on it though and it'll be 6 mo. before stamp is back. $200. But then that sear is truly free of that gun regardless of whether certain other parts reside on it or not.
3. Do what Shattered / SUDS mentioned....addendum. But honestly I view that as essentially option #1 and getting permission from the ATF for such seems superfluous.
shoot me a Private Message if you like and want to discuss by phone...for info to place in various form 1 boxes, etc.
-HH