That’s as maybe Matt, but try retarding the timing just a little. The running backwards (a way of reversing in some single cylinder hot bulb diesel two strokes) is due to the timing being advanced enough to promote sending the piston down the ‘wrong’ way. Back it off a bit and see what happens.
The Swedish Bolinder hot bulb engine (single pot about six litres in capacity, often called semi-diesels) with no reversing gearbox, could achieve reverse by allowing the engine to almost stall, then operating a separate oil lever to fire just before the piston reached it normal (running forward) firing point. This would then fire, and send the piston back down the bore reversing the direction of the propellor. If missed, lots of blue air and a crash!