Farmall Cub

Looking at a 154 Loboy, the owner says it pops out of 1st gear. All the other gears work fine. He says it stays in 1st if you hold the lever in position. What would the most likely cause be and how involved is the repair?

Could be a bent shifting fork not getting it all the way into gear or perhaps a worn gear. Removing the top cover will reveal a lot. The bent or broken fork is an easy fix. Gear replacement means splitting the tractor.