The two most often cited drop-in replacements are Cohesion 3D ($230) and Mini Gerbil (new model 3 out for $115).
You could get any number of other boards to work with some work including the DSP controllers. The other two are just the most common and are specifically intended as drop-in replacements to the mounting level and are well supported.
There are tons of folks doing lots of good work with K40s, especially with some upgrades. They shouldn’t be underestimated.