Thought i would add my experience with RB engines.
Bought myself a K10 for my buggy as it was a great price with good reviews. I will say now i have only run Novarossi in my buggy but was always interested in RB as they are a very similar engine. So far i am very impressed with the performance of this engine. Its a screamer. It tunes very well, no mid needle but its not really needed anyway. Im running it with an RB 2045 pipe. It produces a lot of power and as a result i have run a bigger clutch gear (15T) as it pulls it so easily, and the engine tunes so well you can use variable throttle for long periods (not full or full brake) with no issues of fuel loading up, or stalling. So now i have a buggy with more top speed potential and more throttle control due to the very precise carb.
I have only run about 9 tanks through it, so i cant really comment on life span, but, i can say it still has a pinch when dead cold between sleeve and piston, which is a good sign.
Comparing to Novarossis is a hard task for any engine, but so far it looks good. My experience with Novarossi has been all positive. Amazing life span when looked after, best i have ever seen. In fact, i run an old Novarossi BX21SBK01 short stroke in an old Ofna Dominator truck still to this day and it holds tune and pulls very hard! Lots of comp. It also pulls a bigger clutch gear than the stock truck came with (i have a video on youtube of the Ofna running it if anyone is interested
Sorry for the long post, but, if your on this thread looking for info on RB engines i feel i got something to add.
Cheers.