I made my hall to SCR circuit off the top of my head in the workshop last night, I'll do a diagram and post it up over the weekend once I've worked out how to use my circuit layout program! Its basically similar to the TIM4 circuit with a 1k pot on the output and using only 1 transistor.

Also most of the transistors and the SCR I've used are different to your circuit Jon due to what components are easily available here in the UK. I just tried to match spec as close as possible when choosing the alternatives.

Thanks for the update, but one question for you or Jorge. I like the simple idea of using the magneto coil for the spark, I have a box of those at home! From the pics it seems you have 3 wires coming out of the hand wound coil. I only expected 2. Also did you literally just wind 50 turns of 0.5 onto the core that comes out of the magneto centre? its tricky too see in the pictures, if you have another picture or two looking end on or from the other side it may be more clear.

Thanks in advance,
Rich