Have you looked at this:

If a Ka you own or are considering buying runs roughly, the following possibilities could be the reason:-
Pushrod wear, where unhardened push rods have been installed. Poor cylinder bock cleaning while machining (leaving burrs). And unexpected wear of correctly hardened push rod cups traced to problems with the adjustment screws. (Ford has now eradicated these problems for new production.)

http://www.honestjohn.co.uk/carbycar/ford/ka-1996/?section=bad

Have you tried obtaining any problem codes?

I will go for injector problems a wild guess .

when a car car flags an injector fault, you have to look a little deeper due to the fact that if the engine detects a misfire it will shut an injector off to prevent damage to the cat but often flag an injector fault !

Excesive carbon on the back of inlet valves can cause a hesitation and misfire at low rpm. I actually used to have a sand blasting unit with a hoover line on it that used to fire wlanut husk and olive stone mix into the inlet port with the valve shut and manifold removed and that used to make huge difference.

Is the car fitted with Ford plugs ? Fords do not like pattern ignition parts ! and if not ford plugs, the ceramic insulator is a different size to the lead causing a spark to jump down the side of the plug under load at low rpm.

I never use pattern leads, or coil packs on cars and always use manufactures plugs, can make huge difference otherwise you end up chasing your tail and costing you more. Find someone who has a scope and knows how to use it and they will find the fault without fitting any parts as guess work ! I lived with my scope and so many garages dont even know how to use one ! keep away from them as they are burning your money ! buy their guess work