Thats a tricky one, Sirio engines can be a litle tricky some times. It realy depends on what you want. At the moment novarossi has the most relable engines especialy on-road 21 and 12 engines. Its realy up to you nova is very expencive but you do get your money's worth. There are lots of engines to pick from the crf is quite good. I pited for a friend of mine at a local cup 2 day race meet and that engine is well capable of running 10 minets. It is very touchy on the needles but it is not the latest version and one of the fastest engines ive seen and outstanding run time form it and there fairly well priced. Just look around and ask lots of question's of what people think of the engines they prefer to run. Best Regards Jeremy.