I, also, have a Senator Rototiller. 5HP Briggs - numbers long since worn off. My father bought it in '73. The thing won't die. Drive gearbox leaks a little, but the dirt slows the flow.
My father thinks that Senator was a off-brand name for Gilson rototiller. We are in the process of hunting down the manual among 30 years worth of papers. If I find it I'd be happy to send a copy. Good luck!