I don't know where you would get information other than in small scraps - probably you could make a start by looking for J.A.P. engine collectors or rotary hoe collectors on the internet.
http://www.donaldantiquerototillers.com/British.htmlI haven't seen a stationery JAP engine in years. A rotary hoeing contractor dug up my yard with one of the 600 cc JAP-engined Rotovators in 1963 - that was the loudest, slowest, thumpingest "modern" engine I'd ever heard at the time.
There is quite a bit of information available on JAP motorcycles, and their post-WW2 racing engines, but I haven't seen anything on the stationery ones. You might start by finding the model identification plates for both your engine and rotary hoe, taking some pictures, and starting the process of emailing collectors.