The KA100 can be run right out of the box. It is the 2-Cycle version of the Briggs 206. The cost is relative to the speed, less than a TaG and faster than a Briggs. It is obviously faster than a Yamaha Can as well without all the clutch work.
Also from what I have been told it looks like your savings vs a tag will be in the rebuild costs. I builder I talked to was saying if you started with a fresh motor he would recommend about half way through the season 4-5 races do a full rebuild and then at the end of the year to set you up for the next season.
We competed in the top 5 all season in USPKS on a stock KA and didn’t perform one rebuild all season. Engine was as strong the final weekend (2nd in the final) as it was at the beginning of the year.
My son raced the full USPKS series on a stock KA also. He was competitive all year and set his fastest relative time in the final race. His lap times in the final race were 3rd fastest of the group. Even faster than his teacher – TJ :)