It could be the spring holding the shift star idler roller in place.
It could be a damaged main or secondary shaft case bearing letting either shaft move side to side
It could be a damaged shift fork shaft
It could be a bent shift fork
It could be a rounded off engagement dog on one or more or the drive dogs.
The first step is to pull the clutch cover and see if there is anything obviously damaged or loose. If there isn't anything under that cover, you'll need to pull the bottom end out and split the cases. If the engagement dogs are worn rounded, you'll need to replace individual gears or the whole assembly. Stabor had a whole transmission he was trying to sell for $45 a few weeks ago.