Constabulary

the reason likely is that clutch motors are way outdated nowadays. On my side of the pond you sometimes find 750w Servo motors for about 70€ (not kidding) but that of course is super cheap. Anyway you find proper servo motors for 120 - 150 € or $ so for me I see no reason not to replace a clutch motor that needs service or maybe bearings with a energy saving servo motor. But that's just me. If you want to invest work time and maybe money into an old clutch motor go a head. I would not do that. Just my 2 cents.

its a 31-15 not 30-15 and is a tailoring machine not a harness machine (except for Barbie & Ken horse harness - maybe **kidding**) and what is the number? looks like the check spring in the tension unit is not original and the tension unit seems to be set up incorrectly. I´d get a new spring or complete tension unit. And you better download a manual. Neither 134LR nor 135-34 is the needle for the 31-15 except someone set it up for one of the two. But you have to figure that, hard to tell from the distance. The original needle was / is the needle DBx1 / 16x257 Maybe this helps: https://leatherworker.net/forum/topic/74484-what-needle-size-do-i-need-for-my-singer-31-15/ https://www.google.com/search?q=needle+singer+31-15+site:leatherworker.net&client=firefox-b-d&sca_esv=1cdd3e2116a3bced&sa=X&ved=2ahUKEwi1ndrztsiLAxUF-gIHHUjaKFIQrQIoBHoECBUQBQ&biw=1600&bih=719&dpr=1

The the friction between leather and the rising needle lifts the presser foot. I´d increase the foot pressure first. If it does not help look for a heavier spring for the foot bar.

The Seiko TE-6 head section is pretty much the same as on my former Singer 34K5 (Singer 16 type so to say) which I have restore and later it had an accident and I had to replace the needle bar. I have used a Singer 17 needle bar which is the same as the Seiko and it worked. I used 135x17 needles though the shank is thicker as on Singer 16 needles - it just worked with the new needle bar back then. Just my experience but I no longer have this machine I have replaced it with the Singer 51w. I would simply try a different needle and adjust the needle bar height. Just hand crank it and see what it does. https://leatherworker.net/forum/topic/66729-singer-needle-bar-i-could-use-some-help/#findComment-434368

Google: https://www.google.com/search?client=firefox-b-d&q=needle+singer+51w

ebay: https://www.ebay.de/itm/152425997655

not sure what you mean with needle hook clearance? You mean the distance from the needle to the hook tip? If you mean the rise from the BDC to form a loop then 1/16" is not enough. Machines with 328, 214 or 216 needles usually have a rise of 3.5mm - 4mm from BDC.