I recently did the Sinister Secret of Saltmarsh (well, first half) for my 5e group. I converted it on the fly as well, no pre-conversions. It was pretty easy to sling out some DCs on ability checks for the various obstacles in the book, and just sub in the right stat block. I did replace the centipedes in the kitchen of the haunted house with a swarm of rats, however, because the miniature I have is cool looking and I enjoyed the idea of a swarm of rats piling out of broken cupboards and kitchenware.
The PCs didn't like Ned. Ned is now dead.