Do you have a scrapped block you can cut up? That way you can look at all the dimensions you need to. Porting is a tricky thing, and there are many, many theories about what works best. Unfortunately this means there is no set formula for you. You will, like the rest of us, through experimentation, trial-and-error, and overall racing experience come to draw your own conclusions.
There is no plug-and-play in mower racing.
