FYI: Over in the Yamaha Aircraft Conversions Facebook group we have William Langford who is making  an Avid Mk 4 engine mount jig for the Apex engine.  He is also planning to offer headers, oil tank, coolant tank, and throttle reverser for the Avid which would take about 75% of the work out of a conversion. The RX-1 also fits in the mount but it has a prop location that is about 2.5" lower than the apex. The mount weighs 7.28 lbs. Williams contact number is 701-651-0122 Williams brother Barak is the fabricator, his number is  520 235 9072