Drummyfish just released small game engine called SAF (SmallAbstractFish), and it supports ESPboy! It is a pretty simple engine, it offers:

• 64x64 resolution with 256 colors, 25 FPS
• arrow keys + A, B, C buttons
• 4 sound effects
• 32 bytes of persistent memory (for save/load)

The advantage here is that a game your write in SAF will run on ESPboy but also on the PC, in the web browser and on other hardware (one guy even ported it to a calculator). This means SAF can server as an “emulator”! You develop your game comfortably on PC and then just upload the final result to ESPboy. The engine is completely public domain under CC0! More info in the repo: SAF 1