Fantastic work. One of the only jam games I felt compelled to try and 100%. Didn't quite get there, got all the powerups and 10/12 hearts. Searched around for awhile but not sure where I missed them.
Level design is great, visuals are nice, powerups are solid and given out at good intervals. Backtracking felt good.
If I had to nit I found some of the save points a bit far apart at some frustrating parts, but it was made better when I realized it din't completely wipe items you picked up.
Awesome work on the custom engine too. Interested to hear more what you used for rendering. Did you use an external map editor?