Doc, isn't that like 4.55 gear ratio, like happy time engine and an 11-tooth rear sprocket?!
totally undergeared. in comparison with a happy time engine and 36-tooth rear sprocket, your engine would need a 165-tooth rear sprocket, the size of a GEBE sheave rear pulley.
JMO, you definitely need a jackshaft with extra gears, and that 50-tooth sprocket is unusable. to get it to work, you'd need a 33-tooth sprocket connected to the 11-tooth engine sprocket, which calculates to 3:1.then an 10-tooth sprocket chained to the 50t rear gear factored to 5:1. your ratio would then be 15:1, which is what happy time engines have with a 36-tooth rear sprocket ratio(14.94:1.and that would be with a 33t sprocket spinning next to your left calf, if you could find that much room near the downtube.)
unsure what your usable rpm range is with your lifan engine. if, for the sake of discussion it had the same range as happy time engines, you could get by with an 11t engine sprocket chained to a 15t gear at the jackshaft, then an 8t sprocket to a 72t rear sprocket. this gives you overall ratio of 14.85:1, which compares to happy time's 36t rear gear(14.94:1).
Myron