My DT400 seems to generally have a lack of power.

I have been told its probably the crank seal , is there an easy way to sell if the seal is no good / letting in air.

Thanks

aside from pressure testing I think you have to go back to symptoms 
take the silencer off and see if this helps...could be a plugged exhaust
when I had air leaking L seal and manifold gasket the engine would not idle, air screw had little to no effect, had run-away through the roof RPMs even with ignition switched off.  A lot can change in 50 years so if you recently got the bike you might want to take carb apart and visually verify that all jets are spec (I didn't once and had a 140 main where a 180 was specified and melted piston) and clean.  Also check the timing.  my two cents, all its worth  good luck!

Everything said above in post #2.
I'll second the exhaust plugged theory, especially if it's running ok & just lacking power. Timing is pretty much set in stone on those unless it's broken the flywheel crank key, in which case it's still strobe right [to the marks] but the piston won't be in the right place.