Bit of maths......
Bear in mind that the main impact of the restriction isn't the max power but the max power/weight ratio. This rules out anything remotely powerfull in the dirtbike world.
0.2kw per kilo maw power ratio = 0.268ish bhp per kilo max.
ktm 125 is roughly 100 kilo and puts out 30bhp+ which = 0.3 bhp/kilo ie not legal.
if you wanted a 40bhp bike it would need to weigh 150kg approx.
You might find a 200 exc - gs autolube model (basically a detuned exc trailbike) puts out less than 26.8 bhp at the rear wheel (using a knobbly tye with low pressure will sap 10% of power
) and this weighs 100kilo ish.
You aren't going to find arm wrenching power within the legal license limits (which is the point of them!). Some bikes you may regard as slow are likely to be beyond the scope of your license.
If your looking for arm stretching powerbands your better off avoiding greenlanes altogether, as well as council land. There's plenty of opportunities for enduro's, practice days, mx treacks and other off road competition/facilities where you can legally ride powerfull bikes at full pelt. It works out cheaper than riding a bike your not licenced for and the assosciated fall out from being nicked (let alone the problems it causes legal users of green lanes).
Non road legal mx'ers (such as the rm125) are generally cheaper than road legal bikes. the money saved can go on a trailer/car to get you to the competitions and practice venues.