I don't know what the low-end/second hand market for welders is like in Suriname, but in Europe a welder like that is worth very little even if it works (especially with the low price of introductory inverter welders being sold these days).
I suppose you could supply it from a unit based on a triac, and calibrate it based on its performance with particular sizes of welding electrodes. This is more or less the way they are used anyway, rather than the amp scale being taken as accurate. However, you have the cost and trouble of making the control unit and there may be problems to do with this being designed to work with the magnetic shunt.
I think it makes more sense to find a way to fix the magnetic shunt holder, or look for a used welder of the same sort, rather than mess about trying to find a way to control it without the magnetic shunt.
Mostly people use one size of electrode, for a 100A machine probably 2.5mm, and don't vary the current much. A possibility is to find a position for the shunt which works with the electrodes you normally use, and fix it in position. This is not ideal, but it might do.