Think there was a 'evaluation' 400 big horn in the US there is a pic floating round on the net, looks just like a 350 believe it or not. Don't think it ever went into production.
HP will depend on the fuel you use and what you use it for, but 50 disc valve HP is easy done. He doesn't ride it much since he got his TZ750's but Dave Crussell from the US had 51hp at the wheel from his Scott Clough tuned Big Horn road racer. Was still a mild state of tune.
I'll have to pull a crank out and have look to see if they could be stroked the cylinder can go out a bit so 370-380cc might be do-able.