From memory the Volvo's Bosch coil has a built in ballast resistor, which is needed for a normal Kettering ignition so it does not burn out. A capacitive discharge ignition (as far as I know that is what you have) does not need this ballast resistor, and actually would cause a loss of performance. There are other factors as well, like breakdown voltage, inductance, leakage reactance just to name a few. Go with the coil that is specified/approved.

Another reason why the spark might be faint, is because capacitive discharge ignitions (CDI) store energy in a capacitor and then dunps it into the ignition coil which acts like a stepup transformer. Normal Kettering ignition stores energy in the coil (from there the dwell time requirement to build up energy, not needed for CDI) and transfers the energy from primary to secondary (rather than dunped into the primary). Over time the capacitor in a CDI system wears out and so the spark energy suffers. The capacitor can be replaced, but few general component suppliers stock pulse grade capacitors and it is hard for general public to do this. Consumer electronics do not last more than 5 to 7 years before performance change, industrial electronics go for about 10 years or so before change sets in.

How do I know all this? I'm an electronic engineer working at a company that manufacture world leading electric fence energizers which are effectively just slow running CDI with lots more energy.