I have no idea what a Lintex Viron looks like but I'v rode bikes for 40+ years,squeak,crunch,vibrate,loss of speed/power,sounds like your rear wheel is out of alingment,put it on the stand and see if the rear wheel rotates without rubbing on the frame/swingingarm.If the engine/wheel are all one unit(scooter style)check the engine bolts arn't adrift and the engine is bolted square in the frame.Good luck,keep biking.
Rich
July 2011