ORIGINAL: HellcatAce
Just go for it! If you can confidently tune a 2 stroke, then give a thumper a try. I've had OS and YS and tinkered with friend's Saitos and believe what people say in terms of reliability. The Saito and OS are 1,2 respectivley. The YS engines provide incredible power, but at the tradeoff of finicky tuning. Once you get one dialed though...they are knida bad to the bone! OS has a really cool vavle adjustment tool kit that works on most engines and is worth every penny. As long as you don't try and run too much nitro or get them too hot, you'll find nothing more that a quick check of clearance is all that is required for the valve train.
One other thing that I don't think I read was the fact that a 4 stroke is much easier to make scale or custom exhaust systems for since they don't require an expansion chamber like a 2 stroke.
I have found what is #1 and 2 are more about what the local hobby shops sell. In my area that is Saito and magnum but the magnums once used usually don't get bought again. The Saitos are alive and well in my area though. None of the local shops stock the OS four strokes on a regular basis so there aren't as many of us using them.
Another question I just have to ask is what is different between tuning a YS and any other four stroke? I keep seeing people that have never owned a YS state this over and over. I really would like to know what you think is so hard about a YS and maybe I can inform you about the problem. The older YS engines were/are a balancing act between the high end and regulator that some people were never able to figure out. An inquiring mind really wants to know.