Because there are no emission test for motorcycles TFL (ulez) are only interested in what the manufacturer's Nox levels are from new, so, if you have changed your exhaust system, removed emission control or anything else it doesn't matter.
D16PJM,
The Ulez zone starts in April which is the same as the current congestion zone (central London) and in two years time the plan is to extend the zone out to the north and south circular.
You have two years but from what I've heard once they extend the zone all within with non compliant vehicles will be given a Grace period of maybe another couple of years to change their vehicles.
What part of Sidcup are you from ?