There is nothing wrong with the stock tonearm and it’s much more versatile than Rega. Actually rewired stock tonearm is good, adding fluid damper is also good idea. Cartridge adjustment on the stock toneam is super easy because technics provided plastic overhand gauge and tonearm has VTA on the fly, counterweight is also easy to use (one the arm is balanced the VTF digits on the counterweight are spot on). This arm can be used with many headshells.
If you replaced the arm with Rega arm then you need a protractor to set up a cartridge, Dr. Feickert is the best protractor for reasonable price (a must have).
Jelco is better arm for Technics, but not the only one that can be used instead of the original.
Jelco effective mass is higher than stock Technics tonearm.
Jelso is not designed for high compliance MM cartridges.
Anyway, i think the Jelco alone cost nearly 50% of the brand new SL1200GR turntable which comes with a better arm and much better everything including thenew DD motor (all for $1500-1600 used).