There are several options for drop-in replacement.
The common ones include:
Mini-gerbil
Cohesion3D
Omtech K40+; I believe I’ve seen a newer Omtech GRBL based card but can’t find a link to it
There are a bunch of others. Since your machine doesn’t use a ribbon cable you could use any of those cards.
Monport is newer to the scene with some options.
There are people that use more generic cards and adapt it for use in the K40 but typically bit more work in terms of wiring and possibily having to swap out limit switches.
This sounds like the same issue we’ve seen from another couple of users where instead of using 5V logic the board is using 3.3V logic which just won’t work properly with the machine.
However, based on what you’re getting this should be enough to get power modulation from the LPS. You’ll just never get full power.
This Topic is worth review in terms of the 3.3V vs 5V. TLDR: ask for a new board without the issue.