i'm kinda anal when it comes to engine building. i always pull everything apart and replace all the bearings and seal. then give every part a deep cleaning. make sure there arnt any weird shit going on. (chipped teeth, 40 yrold garbage, or whatever the PO may have effed up.
i'm not familar with the 25mph cylinder but theres usually plenty of meat on the cylinder to open up intake exhaust and transfer ports. i never had problems with stock head..but it's better to turn it than mill it. (edited)