Replacing corroded class 250 or 300 black/malleable iron pipe & elbow on WGO-4 series 3

Hi all, I need to replace, what appears to be, a heavily corroded class 250 or 300 malleable iron elbow and section of 4ft, 1-1/4" black pipe near the return of my Weil-McClain P-WGO-4 series 3.

1. Can I use class 150 fittings/pipe? I can't easily find class 250 or 300. The manual doesn't seem to specify. Looking at an Anvil cut sheet suggests I can go up to a temp of 250F at 225PSI with class 150 fittings. The max temp I've observed on my heat gun exiting the boiler is ~190F, but I know the boiler is firing above 370F.
2. The manual suggests front section B1 return piping has a 1-1/2" pipe size. My system looks like it immediately reduces to 1-1/4" at the return with a bushing (all the pipes measure 5-3/8" circumference…~1.71" diameter.
3. I'm getting some used Ridgid tools (die head, ratchet, reamer, 3-wheel cutter, pipe stand). I'll have a retired machinist (dad) who installed/maintained his own boiler helping, but he prefer sawzall and hasn't done this in years.
4. Any watchouts for draining the system?

See images below.