I shouldn't think there will be anything "custom made" for the Saitos. They may make their own but it won't be any better than fitting a hatori/bolly/es etc.

Any of the 120-140 pipes will do fine. Note that with a four stroke, the pipe is not much more than a large volume muffler. The larger it is the less back-pressure you will get so it will be less restrictive and quieter.

The bolly pipes are good but are quite hard to get hold of - certainly here in the UK

James