Goblinoid Games has released a Planet of the Apes type game called Apes Victorious. It uses a system compatible with Labyrinth Lord and Starships & Spacemen. You can play astronauts or apes or underdwellers. You can pick the PDF up at Drivethru for less than $5. Or, better yet if you are unsure about it, you can pick up the free, no-art version from Goblinoid's website. Grab it here.