I don't think it's a problem with 5th gear - more likely the engine is well down on power. My Dual Twin will pull to 65mph in 4th gear, and gets well on the other side of 70mph in top with 300lb of me sitting upright. The Dual Twin is high geared, but it shouildn't effect the speed that much.
See if your bike will easily reach 60 in 4th. If it won't, the motor is sick - maybe it needs a thoruogh tune-up (particularly plugs, points and ignition timing). If it has good speed in 4th but won't pull in top, maybe you have a binding brake, or a soft tyre.
Graham