I would investigate further, as suggested, before condemning the bottom end. If it's a top end issue they aren't too terrible to R&R valves, rockers, etc.
Either way, if you truly feel as you do for your KJ, my recommendation would be to rebuild or replace the VM. You love it, you know it, it works for you. However, everything else still has over 300K on it and won't last forever either.
Me, if I like a vehicle and it serves my purpose I keep it until it rusts away/gets totaled/etc (I only have 10 right now) for the reasons stated above. If you replace it with another you don't really know what you are getting, what the PO was hiding or neglecting; it could be a while until you are as comfortable with it as you were your KJ.
I'd rather spend the bucks on repairs and upgrades while factoring in down time rather than payments and dealer layups. But that's me and I'm sentimental too...