PVC Window

Hi All,

I need to replace a wooden crawl space door with a PVC/plastic door. The current wooden door is mounted to a simple 2x4 frame inside the opening.
I already found some who offers PVC windows and door in Toronto for an affordable price. But I have some doubt's regarding the frame I’d like to attach the new PVC door directly to the masonry. There is a metal support plate under the first row of bricks above the opening. I’m thinking the 2x4 frame was purely for the original wooden door attachment and does not have any structural load on it.