Had the same problem with my MTD Yard Machines 5 horse tiller. I called MTD at 800-800-7310 and with the model and serial and such off the engine - which are stamped into it - a nice lady was able to narrow down my tiller year. From there with horsepower details she narrowed further to one of two models that I could look up on the MTD corporate home page and get a schematic.
Hope that helps.