Wow, that is a real 'roach-coach!' I expect you're going to have to invest more $ than that bike would be worth. Downtubes, fork seals, rear shocks, seatcover, re-paint, front brake master cyl, front caliper rebuild, rear brake-shoes, a modern headlight, treating the gas tank interior, air-filter(s), probably brake, throttle and clutch cables. and whatever the engine needs. Does the engine turn-over on the kickstarter? Have rodents torn-up the wiring harness? Thank goodness those 'cool' pull-back handlebars are salvageable. Just kidding on that last one!
I suppose by doing alerts on ebay you may be able to find much of this stuff used. Race-Tech and Forking by Frank are two places to check for new downtubes.
I think you could have a very interesting story there, returning this bike from the grave, and so-doing at the least-cost. Take lots of pictures, and keep a file of your expenditures, including a time-sheet.
Here's a simple tool you can use to remove the master cyl piston from the master cyl if it doesn't want to come-out and is stuck.
https://www.vmaxforum.net/threads/1972-honda-750-4-sohc.50102/#post-502853
A thread on re-working the caliper and the master cyl:
Front brake help
I saved myself a lot of frustration by using a grease gun to force out a stuck piston from a SOHC Honda 750 I have.
In one of the threads I posted, there is a description about TIG welding the contour of the front brake cam where it contacts the master cylinder piston end, greatly-reducing the excess free-play, and making the brakes able to 'bite' more-quickly, There still is free-play, but nowhere near as-much as the way it was. A very worthwhile modification, costing nothing but a bit of welding time.
I'll be interested in your process as you attack the
Frugal CB450 Project.