Hay Pat and Happy Monday to you.
I suffered the same quandary two years ago with my 81 KE55. My first priority was to keep the K as close as possiable to orrigional look, I bouoght it brand new in 1981. So I opted for the 5K motor, looks great as replacement for 4K, slots straight in, no mods required, loves 5 speed box, loves 32/36 DGV Webber - extractors & big pipes. Picked up shitty 5k for $100. and rebuilt myself for another $500. Total mod was $600 bucks + $50. for engineers Cert.
Good luck and spend as much as your passion allows you to.
Ben.