2019 Kx450 trans issues: Fix and sell or fix and keep?

Fix and keep depending on the total hours of the bike.  It'll be tough to sell "as is" and recover any real money on it.  A good condition 2019 could sell for $4500-$5500.  If you fix the transmission...it won't make the bike more sellable.  You'll have a bike with a fresh transmission and it should last.  I've got 160 hard hours on my 2019.   

Try to find a local tech who works for himself,  to do the work for cash,  maybe he will let you assist. You can also help cut down on labor cost if you remove the engine yourself… 

A transmission is not nearly as difficult to fix as you might think. Just go slow, take pictures as you pull things apart, lay them out methodically, and pay attention to small details like the orientation of snap rings, etc. 

Agree with this. 

BTW, do you know exactly what the issue is? Or does it still need to be determined upon inspection?

ETA: Keep in mind there will probably be some other work that would be wise to do while you have the motor apart.

I went through the same situation with my '09 KX450 a couple of years ago.  I fell and somehow knocked off the intake boot. As my bike slid, it sucked sand into the running motor and never ran again.  My bike was worth about $4000 if it was running and it would have taken about $2k to fix.   

I ended up parting out my bike, piece by piece, and sold nearly everything on Craigslist or eBay.  It took me about a year, but I got more than I would have gotten from it when it was running.  I got the bulk of my money upfront by selling the suspension and some other parts.  It's time-consuming but also an option.

One of life's biggest lessons (that I always seem to get wrong):

It comes down to what is your time worth to you.