I remember when I hung the engine on N614EF, we had six well intentioned guys who struggled to get the big 180 horse engine mounted and to come up with a way to get all the bolts to line up; it took all afternoon.
Granted the 100 hp, fuel injected Rotax 912iS is smaller (and $10,000 more expensive) but it took me less than an hour. By myself.
Just had to approach it like docking at the space station. A minor bit of wrestling and twisting but it's light so no big deal. Everything lined up perfectly.
It helps to own an engine hoist.
Van's was pretty explicit to check the Rotax maintenance manual and observe their torque callouts, but I couldn't find one for the firewall mount bolts. In the maintenance manual, all the hardware is metric.
So, at least for now, I observed the torque for a 3/8-24 bolt (160-190 inch pounds) and sent a note off to Van's to confirm. With the prevailing torque factored in (roughly), I torqued to 175 inch pounds, which on the torque wrench was 168 with the crowsfoot adaptor.
I'll make adjustments as necessary