>>1953163Hiten steel bike boom era bike.
Best route is to find another used bike as a starting point, if you have a reasonable used market. It's in suprising good condition. Selling to some hipster and buying something better with that plus your 100 dollars might work.
Fitament is always important. I bet that saddle isn't very good fitting.
Check if the wheels are steel. If they are, you can get a big boost by taking AL wheels from another 27 bike, or 700 c bike and changing brakes. Do this from crashed and salvaged bikes for super cheap. Not worth it to buy new. You might also source old 27 tires this way as well.
>>195317227 wheels. Very limited tire selection. Don't bother with tubeless.