I have a 1964 Allis Chalmers B10 with a 9hp Briggs cast iron engine. I use it to cut grass in the summer and plow my 400ft gravel driveway in winter (upstate NY). It works well, but the old engine is getting tired and it seems barely powered to handle the plowing. I could rebuild the briggs, and that may give me the power back, or I was thinking of repowering it with something like a 18 HP or 20 HP briggs vtwin vanguard or Honda vtwin engine. It seems like I could fit these engines in without too much modification, but would I notice much difference in power? I know those old cast iron engines are known for their torque at lower RPM ratings. Thanks.