Lost Reverse in 904. Please help!

Since you say the fluid does not smell burned and you have all forward gears (I assume you verified that you do in fact have 3rd gear, not just 1st and 2nd) that indicates that both forward clutches should be ok. As Trailbeast said it's possible it's a rear servo problem. It's also possible the trans. has never been serviced and the rear band is just way out of adjustment. When you drop the pan look the rear servo over good and if everything looks like it's in place check the band adjustment. I first grab the servo lever by hand and see how much free play there is in it. Shouldn't be a whole lot. If so the band needs adjusted. The manual says to torque it to 72 inch lbs then back off 3-1/2 turns. If you don't have an inch lb torque wrench there's an alternate way you can do it. Back the adjuster locknut way off and tighten the band up snug so you can't even begin to turn the rear drum by hand then start slowly backing off the adjuster. At a point the drum will free up so you can spin it by hand. At the point where you feel there's no drag at all from the band back it off another 1/4-1/2 turn and tighten the locknut making sure the adjuster didn't spin. Good luck