Replacing Drum trap

Hello, I am having to replace this existing trap with a PVC trap in my house. I am not versed in plumbing as I am a Carpenter. Please help me to understand how I am to transition the existing drum with PVC I have not found a system that would work with current layout. Thanks in advance!

 

If you put in a P-trap, it will need a vent on the trap arm.

 

Why does the trap have to be changed?

You may already have taken care of this and I rotated the picture to make sense out of it. Is this drum and pipe made of lead or tin and it appears to be the bathtub trap? The pipe to the right leads to the drain, the left pipe is the vent and the bottom one is the tub drain. There are plenty of pictures and you tubes on how to a tub is to be connected. What may be difficult is to re-plumb for the vent since it is not known what type of pipe it is and its inside a wall. Depending on what type of pipe you'll need to connect, Fernco Shielded Couplings may have to be used to transition from new to old pipe. From the pictures the waste line is high near the floor. A PVC trap may come down about 6-8" from the floor, then there maybe a slope problem to connect to the main waste stack.