There has been some discusion of this topic on the UK Carpeneters Fellowship web forum. Look at "worm drive saws / Prazi beam saw". There is also some talk about the Mafel carpenters chainsaw which does a similar job, slightly better for much more money.

You might also want to look at the discusion there about "13" saws" too, as my advice for cutting larger timbers is to use the method you are and perhaps consider a larger circular saw (10 1/2" or 13" or even 16".