Seeing as how no one else has brought this up yet, and this exactly fits the description of OP's request, and there is no separate thread for it, here it is!
Classicube 1.3.7, a Minecraft clone written from scratch in C, fits the bill of what you are looking for. Ported to MorphOS by BeWorld, Classicube 1.3.7 requires SDL 2.32.10 and TinyGL.
Somehow I found this thread by accident and thought that something like this was recently uploaded to morphos-storage.net. Turns out, it was!
Mac Mini G4 1.5 Ghz, ATI Radeon 9200 64MB, 1GB RAM