81cm?!? thats ~32" there are people riding 36ers with shorter inseams than that.
Again I don’t think you will have a problem. Building a theoretical unicycle in my mind you would need 15" for the wheel, 6" for 150mm cranks 4" for the frame and 3" for the saddle = 28" so I guess you are close and may have to cut the frame a bit or use shorter cranks. I would save the frame cutting as a last resort as you loose adjustability.