I have never had any issues with Dover. Ran his flatheads back in the 00's. Many good wins. Took a long break from karting, and came back to it this year. Shopped around and was surprised to see Dover engines priced much better than most. Bought one of his complete clone engines, and I can tell you its not down on power. After getting things figured out from a 9 year break, we are back at the front on a local level.
Also, everytime I have ever called them, they are more than happy to talk for however long you want. I definitely would not shy away from his motors just because they are priced lower than most.