Hi Simo !
Yes, the exhaust is exercising me somewhat.
My plan is that is going to be a long term project, keep me out of mischief for a few months, so exhaust is on the back burner at the moment.
In an ideal world, an SL175 system would go on. However, these are rare and expensive, and I doubt that it would fit the CB frame without extensive mods, so it'll probably simpler to build ( or get built ) a custom system. Also, for an off road bike, it seems mad to run the pipes under the frame. Despite the stock bash plates, used pipes I've seen on line are often well dented.
I don't want to go for a high level CL type system. Pipes will be in matt black heatproof paint on mild steel, not chromed or stainless tube. So, bend some 38mm tube to make a 2:1 system similar to the SL, with an silencer can in the SL position is one possibility.
Or, perhaps simpler but heavier, something like the SL350 system, with individual exhaust pipes and a can on either side. That could be achieved using the stock CB down pipes, extended with 38mm tube with a 30 degree mandrel bend to flip the end of the pipes up, into silencers mounted in the SL350 style.
For silencer(s), I'm looking at Triumph Street Triple 675 cans. These are available very cheaply on Ebay, many owners ditching them in favour of noisy straight through 'race' exhausts. No catalytic converter in them, I won't get caught like that again.
( I fitted a Triumph Scrambler can to my CL175, couldn't work out why it was getting too hot to touch, until it dawned on me that the internal catalyst was struggling to cope with rich carburetted exhaust gases ! ) The Street and Speed Triples have the catalyst in pipework closer to the engine.