Having driven most all these engines in bikes, I can tell you the KTM 200 is clearly the strongest by a long measure.
The KDX220 is not even up to par with a KTM 125 in stock form, but good mid-range. gearbox is a bit wide 1st-2nd.
The DT200 more of a screamer than the KDX but still less than the KTM 125. Very little mid range even with the powervalve.
Stock DT gearbox has 1st too low and 6th too high, the Blaster gearbox is better.
I am running a KTM 250 and biggest problem I have is the narrow 5 spread gearbox spread. Power is ample.
I sized a KTM 125 (same case as the 200) to fit and it fits in a Blaster chassis with room to spare, much better than my 2003 250.
The KTM200 comes in 6 speed gearbox in close and wide ratios. I prefer the wide ratio but for MX work the close may be better.
For what it would cost to sleeve down any MX engine you could buy a good KTM200.
Any year from mid-1990s to present would do. Hydraulic clutch came about 1999 and power and mid-range torque increased very slightly into the mid 2000s.
Most of the power increase was head and cylinder which are readily interchangeable.
All year KTM200 are good. Can be bought for $400-$1200 and used pretty much without any needed engine modification.
Steve
