I'm studying the on-line parts breakdowns here and jogging memory to try and get a feel for what has happened.
I suspect, since you've already eliminated a linkage adjustment problem, and due to the suddenness of the problem, that there is a problem with the internals of the clutch housing. Some folks call this the "reverser," and think that if the machine operates in reverse, that it eliminates any problems with that. But on this model, that unit controls both forward and reverse.
On the Syc-shift, there are two side-by-side counter-rotating shafts with clutches, power transmission is shared by matching and intermeshed helical gears. The selector selects which clutch to engage.
I'm suspecting , due to the noise you describe, that the machine has lost a shaft bearing, and it is side-loading the the forward shaft when driven forward, which causes the machine to bog down. The noise is the shaft, bearing, and helical gears spinning out-of-true.
You can drain the oil and look for metal particles, larger pieces would probably confirm that you lost a bearing or had a clutch come-apart. You can remove the filter and cut it open and also look for metal particles, these will be smaller. Sometimes with a bearing failure, it can cause major damage to the shafts and gears inside.
I don't know if this is something you want to try and repair yourself, but if so, get back and either I or someone who is more familiar with this specific model can help you through the process if you need such.
I can also provide you with a link to the on-line parts diagrams if you need it, they can help you in determining how it works, what could be wrong, and if this is something you want to take on yourself.