My speedo has dropped off but dose come bsck up as do my revs now my engine light has come on

Riding without a speedo is not legal.
If a speedo is fitted, it must work but if you ride sensibly, I expect you will be OK.
My XLCH 1000 had a rev counter and no speedo.

Without the make, model, year and engine type and size can't help you

Don't have a lot of experience with this model , but , sounds like rev limiter is engaging because computer believes vehicle not moving maybe ?
Is speedo working if electric ? if speedo geared then is speed sensor bad , I have seen that before .
Hugh

What's the color of the smoke? white, white with a blueish color, or black?

I have a similar problems on my 2005 9-3. It has developed a number of minor electical issues. including dial and false warning light triggers. I have fixed most of them by simply going round the car disconnecting the electical connectors and spraying "switch cleaner" (Maplin or Radio Shack).
The only problem I have left is the rev counter occasionally starts reading high, removing the fuse (end of dash panel - 4th fuse down) for about 30s with engine off normally resets everything back to normal.
Hope that helps.

My 04 Ford Focus the battery light keeps coming on & going off, have fitted a new alternator belt but it didn't solve the problem. The battery connections are good. Also the rev counter, speedo & mileage clock keep going haywire when the battery light is on. Do you think it is the alternator? Or is it some other common problem?

Could be the cam shaft sensor fault.

Its very probably a bad circuit board behind the Rev guage and Speedo. Before you replace it check that the wiring harness is connected firmly.

speed sensor on the back of gearbox

HAVE YOU TRIED DISCONNECTING THE BATTERY & RECONNECTING THE BATTERY THIS SHOULD RESET REV COUNTER.