Nick, I've had a quick read of the constructional article

http://www.geotech1.com/pages/metdet/projects/hammerhead/HHv1p5.pdf

and it's a lot clearer now

So, there's an astable 555 running at 900Hz - 2300Hz with adjustable
duty cycle. This pulse train drives two 74HC221 monostables, which
in turn each output a delayed pulse with adjustable pulse width

The short answer is that this can be done fairly easily with the 628
using code suggested so far. PWM will work to replicate the original
555 train but as the other two need to be done in s/w you might just
as well use s/w for all

In the schematic

http://www.geotech1.com/pages/metdet/projects/hammerhead/HHd1.pdf

the controls are analogue (pots). It won't be possible to emulate this
exactly using pushbuttons without some disruption to the output until the
new values come into effect but in this app I don't think that's too much
of a big deal. OTOH the pulse parameters will be more consistent and
precise with a digital generator

My thought would be to use B0 INT to break into the pulse train. Push
buttons would be diode-OR'd to generate an interrupt from any button
press, with each button also going to its own PIC input. These inputs can
be polled to determine which button caused the interrupt and which
parameter is changed
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