The box is an air chamber. The YS is typically described as super charged and pumped. The 91fz is the later version, but very similar to the 91ac.

Go here to download the manual on the 91fz
http://www.yspower.co.jp/pdf/fz91-e.pdf

It is safe to use the fz manual to operate the ac

Edit. The case on both ac and fz has ac cast into the case. There is a minor difference within the airbox that sets them apart. Your engine could well be the fz

Argueably, the YS 91 puts out about the same power as many other 1.20 size engines. Typically it is run on YS 20-20 fuel, but will handle 30-20 heli fuel without incident.