Easiest mod for low rev torque MC65?

[bergerhag]

My kid has an MC65, very nice and powerful bike. But we mainly run enduro/trail riding, so the powerband i a little too peaky.

What's the easiest way to shift the power to lower part of revrange?

I was thinking of putting in an fixed exhaust valve, but the cylinder has a three port exhaust, which makes it pretty complicated to fix. Is there any chemical compounds that would last the temperatures in the exhaust? Like JBweld or smth?
Or should I change the exhaust pipe?

Any other ideas?

So come out all you twostroke tuners