An air fork will be a huge upgrade for that bike. It’s much lighter and provides softer landings and more controlled rebound. I think that bike is a straight steerer tube and 80 mm travel. It might be tough to find an air fork with 80 mm travel, but you could probably go up to 100 without changing the geometry much. A Rockshox Recon Silver like this one might work well. https://www.amazon.com/gp/product/B0857DR1J1
With the threadless stem converter you lose the part of the old stem / where the brake cable goes through it. So you need something like this Tektro Front Cable Hanger Black... https://www.amazon.com/dp/B006GHDRYC?ref=ppx_pop_mob_ap_share which I put where the front reflector was. I think there’s one other option
Not to complicate things for you by suggesting yet another rust removal method, but Nevr Dull has always been my silver bullet when it comes to rust removal and one can will last me ages. For extreme build up it may not be the fastest or most efficient solution, but it's one of those thing I stay stocked up on like duct tape, etc. It's always handy.
I have a fully rigid steel bike and I would totally slap a newer Rockshox fork on this bike to replace the old one.
Something like this Recon would be a lot of fun.
What bars are those? I’m rocking some ridiculous BMX bars and stem I had laying around. Looking for something with less rise, but same style. Maybe something like the fairdale MX if I can find a black one.
There's a two-bolt faceplate gooseneck design on Amazon for $20: https://www.amazon.com/Fito-Aluminum-Bicycle-Compatible-Mountain/dp/B08CMR5PDV. I used it on two different bikes. Looks rough, but seems to do the job.
That’s a lot of cash. I found these on Amazon well rated- Fooker brand. Had to get it after seeing that branding. https://www.amazon.com/dp/B07C5NLL3M/ref=cm_sw_r_cp_api_i_J117T78XCEMK5JV48FHB?_encoding=UTF8&psc=1
I used an Origin8 Quill Adapter, 22.2 to 28.6 to adapt my Hardrock’s 1” threaded fork to a 1 1/8” threadless stem.
Because the adapter has a quill, and the threadless stem bolts to it, the spacers were completely ornamental. I just added a bunch to fill up the space, after determining how high I wanted the stem to sit.
Maybe it's not the best looking ... but we put one like this on a friends RockHopper, and it's a pleasure to use. You don't kick your chainstay instead of the stand, and it doesn't bang into the crank. Highly recommend if you're getting on and off your bike a lot.
I replaced the bottom bracket and crankset with a cheap one from Amazon. BB fits properly and the chainline with an 8 speed feels right.
GANOPPER 170mm Crankset 30T 32T 34T 36T 38T Single Speed Mountain Bike Cycling Road Bike Crank Arm Set https://www.amazon.com/dp/B08TQWNBS8/ref=cm_sw_r_cp_api_glt_fabc_E64BBZWSWDGB9VQ2ZMXD?_encoding=UTF8&psc=1
Just this GANOPPER MTB Crank Arm Set 104 BCD Square Taper Mountain Bike Crankset 152mm 165mm 170mm https://www.amazon.com/dp/B08HSMCRJB/ref=cm_sw_r_cp_api_glt_fabc_BPDH84HA2XWG0EVYEMWJ?_encoding=UTF8&psc=1 and chopping the chain to new length.