Technics magnesium tonearm 1200G vs Jelco 850

I doubt the Reed is a drop-in replacement for the OEM tonearm, but it prossibly could be done if one were willing to modify or replace the tonearm mounting board.  And of course the Reed 3P is superior.  So too are a dozen or more other tonearms.

In my experience, it is usually more about the arm and cartridge combination than it is about the arm and turntable combination. In other words if your cartridge is better mated to the standard arm on the 1200 G, then you will hear everything as sounding better than if you were to substitute another tonearm like the Jelko. Conversely, your cartridge may mate better with the Jelco. The only way to choose is by trying both. On a pure quality level, I would guess the 1200 G tonearm is probably at least on par with the Jelco, if not better.