I don't know a trick, but what I did with my 50 that had been setting outdoors for about 20 years and was "stuck" in neutral was remove the floor board panel and take the top off of the trans. The shifter mechanism is all in the cover. There are 3 shifter rods or shafts, one for 1 & 2, one for 3 & 4 and one for reverse. Each has a yoke or fork that slides the gears forward and back. The reverse fork is much smaller and engages a lever that slides the reverse gear. 1-2 and 3-4 shafts have 3 detents or grooves. Above the shaft in the cover is a steel ball and detent spring for each shaft to hold it in place... Probably the detent spring/ball or the shifter shaft is rusted, or stuck in place. With the cover off you can inspect (you can't really see the springs and balls) to see what's rusted, stuck, damaged - and just clean and lube everything with diesel, penetrating oil, WD-40...and get all the shafts sliding back and forth. With cover off you can inspect the gears themselves and slide them back and forth. Maybe drain and refill the oil while the top is off. Be sure the gears are in neutral and everything is lined up and put the cover back on. Good luck.
