Aye TimB,
If your oxygen sensor is duff, the ECU cannot manage the injection of the correct amount of fuel into the cylinder! Hence the black sooty exhaust, excess fuel, explosions from unburnt fuel igniting in a hot exhaust and difficulty in starting.
Cure? Buy a new oxygen sensor to enable the rest of the standard ECU/ignition/fuel injection system to work.
If you wish to modify the exhaust system and run without an oxygen sensor, then you are going to have to use an alternative to the current injection system. More dosh!

There is no need to give your exhaust a "DYNOROD" treatment, that archaic practise comes from the days of engine oil/petrol, 2 stroke premix and pre Suzuki and Yamaha oil injection systems of the late 1960's, early '70's.
Get the oxygen sensor sorted out and then start using the bike. "Decorative" bikes slowly corrode away.

So far this week I have put over 250 miles under the wheels of the '74 Ducati 450 and over 140 miles under the Yamaha SRX600 mono. They need to be used!

Good health, Bill