You could use a quilted Troxel seat, especially if you're going for a 1970s bmx look.
Lots of choices for laidback seat posts. Most bmx posts were 7/8". You may be limited by your tube diameter. Search "old school bmx laidback seat posts" on ebay for options.
Also, rather than pay huge money for vintage bmx cruiser bars (if you can find them) or use repop stuff, find some used Honda SL70 bars or equivalent. Very 1970s bmx moto. Have used them on a few projects.
I'd also score a cebe or slimline double clamp stem. They're available for relatively cheap money.
I personally only ride new rubber. There are a few suppliers making a reissue Comp III skinwall tire. They are great in the dirt and pretty good on pavement. They come in 26" x 1.75" for the rear and 2.125 for front. That model is an early 80s style, but you probably won't find a Comp II cruiser tire for less than the cost of a human arm and leg.
Search "Kenda K50 Comp III skinwall tires"
I'd go with 26" Araya 7C hoops laced to Shimano, ACS, Suzue, Sunshine large flange hubs. The 26" Araya 7x hoops are more plentiful (and stronger), but date from about 1980 and newer. The 7C would be more appropriate for a 70s build.
Get yourself some Preston Petty or OGK MACH grips, an Ashtabula, Sugino or Takagi MX OPC.
I'd go with a Shimano center pull rear caliper and a Dia Compe "red dot" straight lever.
Or none of the above. Enjoy.