Duct tape residue. Dont use paper towels on the glass. Use only soft
cotton (old t-shirts). Use one of the window cleners made for plexi or
soap and wter. Scrape with your fingernails if you must.

Seat Belts. Strongly consider putting in shoulder harnesses. They have
saved many lives. There is a kit with an STC for this. You will need an
IA signoff on the STC.

Good luck. A Cessna 150 is a great airplane!