Ikina Circuit is on the Island of Ikina (3.87 km²)
I happened to have stumbled on an interesting map at the Onomichi Ferry Terminal which has one of my favorite Japanese words, サーキット (circuit). It usually means there is some type of motorized racing. And today I rode my bicycle out to Ikina Island to check it out.
To get to Ikina Island you can take the ferry from a couple locations, but if you're a tourist you'll likely find your way by Innoshima. Google maps does not have a location for it, so I've dropped a GPS location in case you would like to go or see the satellite images. The island is quite small and the ferry runs every hour or so, so plan ahead!
Ikina Circuit was founded in 1988 and is purely a minibike track. During my visit there was a top rider from GP3 and a motocycle racer who was teaching his daughter the ropes at age 12! The season is year round and there are likely some practice or race on the weekends. Just up the road is a flat track area. Flat track is not as popular, but there was a school going on that had some great riders!
Inside the Club House (a small shack with memorabilia, track supplies, and many photos of the track's riders and history)
Flat Track Area: