engine will not rev up

It’s very difficult to tell anything from such a short description. If the plugs are ok and the HT wiring is good you still have a choice between ignition and fuel. Does it rev normally in neutral or park? And only not rev in gear?
Timing that’s way out could cause that problem. From memory the 110 engine should be about 30 btdc at 3500rpm with the vacuum disconnected.
There is an electronic spark control device under the hood somewhere – its silver and when defective will cause problems that are very difficult to solve. As can a defective coil.
On the fuel side there is so much: fuel pump not giving enough pressure at the higher speeds, Fuel distributor out of whack but that’s reasonably rare, Blocked air intake – check the air cleaner and filter stuff etc.

It’s a mater of being methodical and going through all the bits one at a time.
If its fuel injection you will really have to get it checked by a Bosch mechanic as so much is dependent on fuel pressures, but it is all mechanical and there are no nasty computers costing thousands like in the new ones.
Good Luck

Peter,
Must admit, these are the kind of challenges I like...
Right let's see:
Not revving up in neutral and not misfiring?
If so, go straight to the spark and spark advance mechanisms, ie check with a timing light if the spark "moves" as revs increase...? I presume you guys have checked obvious stuff like corroded distributor cap contacts/cracks, rotor condition, etc, etc.
Another possibility is whether someone has mucked about with it all?
If this doesn't help you, then we need more specific info like any different symptoms under different conditions, ie hot/cold, park/neutral, service/work history like exactly what has been done or happened just before this started ), etc,etc...
In my experience, fuelling trouble is out of the question here, because if there was not enough fuel to rev up at standstill, then there wouldn't be enough to do even 20mph...
Let us know how you get on

Definately try the fuel filter.... the tanks are very prone to rust and all kinds of cack can be in the tank... I recently changed mine and it made a massive difference. If it works you had better get the tank cleaned/repaired...