From memory the filter only causes drive problems but it could be blocking most of the oil so the hydraulic pump doesn't work either. I would start with the filter.
Lift the cab, on the left side of the block that sits n top of the hydraulic pump, there are 3 hoses, you want the middle one, remove the fitting from the block (the bolt size is larger than the other 2). There will be a spring that is pushing on a bronze filter, chances are it has never been cleaned. Best if you installed a new one but you can clean it with diesel and compressed air if you need to. You may need pointy nose pliers to get it out too.
I honestly would start there, then you know for sure its clean.