Bmw 650X Challenge circa 2007

I am pretty sure the whole frame is aluminum; the bottom castings are welded to the frames perimeter beams, right?

I am not sure it you really meant it this way, but the 449/511 engines are not even close to the same as the 650's

They are race bike engines, with race bike maintenance/rebuild intervals and race bike gear ratios.

The 650's are street bike engines, with street bike maintenance intervals (no rebuilds) and broad gear ratios.
The 650's can be 100,000+ mile engines.

You are actually correct haha. I was thinking G450. My bad.
Is the 650 based on the Rotax engine in the F650s?

Yes, the G650X engine was based on the original Rotax design of the F650's
Similar to our TR 650's now.

Ahh. Got it. Had TE on the brain lol. I owned a 97 F650ST. My first motorcycle in fact.

Steering head and down tubes are def steel. The bottom part, where foot pegs mount are cast aluminum.