Sell it.

Is this the "Shortstar" engine?  If it is, it's the cadillac Nortshtar  32
valve double overhead cam with 2 cylinders lopped off.  Arguably one of the
best engines out there today.  What do you mean by "Valve Slap"?  Not a term
commonly used to describe engine noise.  If this is lifter noise, the engine
has hydraulic lifters, it doesn't mean the engine is toast.  I have an
Aurora with the 4.0 version of the Northstar and for whatever reason all of
the lifters on the front head wore out prematurely and I had very distinct
valve train noise.  Finding it to be odd to have the problem I replaced the
one noisey lifter.  Soon a second was noisey.  I replaced that one, then a
third.  I finally went in and replaced all of the ones on the front head.
that  was over 2 years and 100,000 km ago and it's still perfetly quiet and
oddly enough I've never had any noise at all from the rear head, it's still
running al of the original lifters.  I've also heard it in STS's (cadillacs)
so I know mine was not an isolated case.  I believe there must have been a
quality control issue with the lifter manufacturer.

Bottom line, confirm that it is in fact the lifters, do all  of the ones on
the head with the noisey one and drive it.  The crankcase design and
efficiency in terms of power of these engines is remarkable in my opinion.
I also think your service guys are selling you a bill of goods if they want
to replace the engine because of a valve noise.
FWIW
Steve