Broken rocker arm!

Hi, I recently bought a second hand 2E engine for my Starlet, I discovered that a Rocker arm was broken and has fallen off.
I also discovered that the previous owner has changed the whole crank for one of a 4E engine. The 2E engine has 4 balances while the 4E engine has 8, but all the other things seem to be the same.

Here is the question: What could be the reason/ problem that the Rocker arm has broken off?