If the trike is sinking in mud, not much you can do about that. Your 90 should haul you around though. Heck, I ride around on suspension atc70s and they haul me around, but I have a stock 70 engine in one, and a 125 in another, and I'm over 200#. Have you tried adjusting your clutch? It is on the right side of your clutch cover by where you right foot should be. There is a slotted head screw that is locked in place by a nut. Put a slotted screwdriver in the slotted screw, and at the same time loosen up the nut, till it's pretty much at the outer end of the screw. Gently turn in the slotted screw till it can't be turned any more, gently. Then back out the slotted screw 1\8 to 1\4 turn. Then while holding the screwdriver still in the slotted screw, tighten the nut back up not letting the slotted screw turn. And that pretty much is your clutch adjustment. Now it may or may not help depending on how worn out your clutch is. You could also try putting your trike into its' "Lo" range and see how that does. Remember, your riding an old trike that probably needs some TLC. Also, seriously think about changing the oil in that sucker, it probably needs a good oil changing pretty bad. And a new spark plug wouldn't hurt either. Also, if you have an auto
parts store close, see about picking up a can of "Sea Foam". It will have clean the carb out while you're running gas through it. I'm sure the carb is a little dirty and it can't hurt.