I'd probably dump some mineral spirts, diesel fuel, fuel oil or kerosene in the trans (what ever was cheapest and on hand at the moment)!
Fill it using your judgement to cover enough, but leave enough room for working and take one of those stiff parts washer brushes, and go at it!
Jack it, and block it up, and spin the rear tire once in a while doing your transmission brushing chores.
Maybe work at unsticking the clutch first that way you can spin a tire (with the clutch disengaged) to spin the transmission in each gear!
Use some more of one of the liquids mentioned abobe to flush it out as needed.