The OP has a problem that has been ignored by him and consequently in the answers.

He writes "fire when the signal is gone" which means that the timing occurs on the falling edge. Where I think the OP is hoping to just increase the "charging" time of his new coils he will also delay the firing time. Either by a fixed amount of time or a variable amount of time that would cause very real and ignition timing problems unless they were compensated for elsewhere.

This may be a misunderstanding but that seem unlikely in this case. However the original factory engine management system may be deliberately altering the "charging" period to change the spark energy but this is unlikely, more likely it starts the "charging" period based on some crankshaft angle reference and then fires the coil based on the ignition timing lookup table.

The various delay circuits proposed should work mostly as advertised but may cause unexplained ignition timing changes and variability due to engine RPM.

However as indicated on one comment at High RPM it seems that there will not be enough time to charge the coils this is not the case as a 4 stroke engine needs 2 revolutions per spark so will have twice as long and within the limits.

There may be a remedy and this is to extend the sparking period to say 3ms (or something between 2 and 5 ms I expect) and then switch to charging which may now be much longer at low RPM but close to the desired minimum at high RPM and then use the falling edge to create a spark pulse starting at the correct instant.

The only drawback here is that the charging period may be even longer than usual at low RPM and cause unnecessary heating of the coils but perhaps this is something that is acceptable. The given timing circuits (like @jasons explanation) could likely be adjusted to this kind of functionality but my fuzzy head is not up to the task just now.

The OP has a problem that has been ignored by him and consequently in the answers.

He writes "fire when the signal is gone" which means that the timing occurs on the falling edge. Where I think the OP is hoping to just increase the "charging" time of his new coils he will also delay the firing time. Either by a fixed amount of time or a variable amount of time that would cause very real ignition timing problems unless they were compensated for elsewhere.

This may be a misunderstanding but that seem unlikely in this case. However the original factory engine management system may be deliberately altering the "charging" period to change the spark energy but this is unlikely, more likely it starts the "charging" period based on some crankshaft angle reference and then fires the coil based on the ignition timing lookup table.

The various delay circuits proposed should work mostly as advertised but may cause unexplained ignition timing changes and variability due to engine RPM.

However as indicated on one comment at High RPM it seems that there will not be enough time to charge the coils this is not the case as a 4 stroke engine needs 2 revolutions per spark so will have twice as long and within the limits.

There may be a remedy and this is to extend the sparking period to say 3ms (or something between 2 and 5 ms I expect) and then switch to charging which may now be much longer at low RPM but close to the desired minimum at high RPM and then use the falling edge to create a spark pulse starting at the correct instant.

The only drawback here is that the charging period may be even longer than usual at low RPM and cause unnecessary heating of the coils but perhaps this is something that is acceptable. The given timing circuits (like @jasons explanation) could likely be adjusted to this kind of functionality but my fuzzy head is not up to the task just now.