I am one of the (apparantly) few people that believe an engine manufacturer knows more than I do about the construction of their engine.
I will modify kits and ARFs to suit my wants, but I have built many planes, and have never built an engine. My first gasser, A DLE 30, came with 4 prop bolts and I am very happy to have them. No way am I going to spend more money to get a single shaft adapter. One thing I don't like about my 120 size 4 stroke glow engines is I have to tightening the heck out the prop nut to keep it from slipping. 4 bolts, it isn't going to slip. And why would I want to go with a smaller shaft? For some reason I see nothing wrong with running my engine the way I bought it.
Sure for some, with the tools and experience to modify an engine, or if you are professional contest flyers, go for it, but don't tell everyone else, especially people new to gas that they HAVE to change the engine from the way they bought it.
I also disagree with some that you have to have "Lots of experience" to fly gas. With the planes and engines they have available these days someone could use a gasser as a first trainer. I know people that do. The engines are reliable, the planes fly good. Why try to tell someone they have to make things difficult to have fun?

I guess you can tell I'm a sports flyer. I believe in flying for fun and I don't have to be precise in the way I do it.
I don't mean to sound like I'm running down very experienced people who want to modify their engines for a good reason. I'm just asking, be easy on the new guy who just wants his first flights to be a good experience.