‘02 Nissan Maxima

I purchased a 2002 Maxima back in 2014
with only 130,000 miles on it, no cosmetic
damage, ran great, and I was driving my
bro in law a couple counties over in aug
2015 and set the cruise control because of
an ankle injury and we were on the
interstate and about to go up Monteagle
Mtn we barely began the incline and
instead of shifting accordingly it shot up to
6000 RPM’s and cut off. Once cooled
down we kept a steady pace of 45mph in
the slow lane but once we topped the mtn
it cut off again. He began freaking out and
wouldn’t leave it alone once he finally got it
started it began smoking under the hood
so I yell to turn it off but instead he started
revving the motor up. The car will crank
now but the mechanic said he’s positive
the heads are cracked. I only paid $3,000
for it and I’m not sure if it’s worth buying a
motor at a pull apart of somewhere, but I
LOVED that car and only had it about a
year and it’s been at garage for 2 years
cause I haven’t been able to repair it. The
mechanic said a few ppl have inquired
about purchasing it. Personally I would
love to fix it but should I even bother or
should I sell it for parts and if I go that
route what estimate would you recommend
me selling it for???
I purchased a motor for a Camry at LKQ
before with so many warranted miles and it
turned out great. Please help.