A High speed cutter will melt the plastic as you cut, but unless it is really thick material it will work. You will just have to break off the excess flash,that forms on the edges and be,careful the soft,plastic,doesn't,grab the plastic and hop the tool.
A hack saw by hand is likely the quickest and easiest way. An exacto saw may have trouble with any thickness over about 1/4" via kerf gap is so thin they are prone to closing slightly and grabbing the blade.
The wider cut of a hacksaw won't matter as your extending it anyhow. Heat/flame will darken any lightening of color from working the plastic if that bugs you.
JB Weld doesn't always stick to plastics well, especially if it has a smooth, or oily texture. Sanding won't help much if that is the,case.( Wrapping, trapping areas in a dovetail fashion with a thick gloppy layer is the unsightly but effective work around if hidden)
I would use a wide flat metal bar for firming up the frame and the added weight, machine screws and JBW or other epoxy...maybe wood ... Radio Shacks epoxy was pricy, small tubes, and my old go to when others wouldn't stick I wish I had stocked up. Flexed though, a better glue than filler.)
Wood will flex under a heavy train if thin. It bows it's belly in a droop. The frame here was bent in half, likely by the "Ebay Devil Child' with a bat" (from a thread yesterday) as there was also shattered 6464, broken porch RioG crummy, the jagged side of an SP caboose, and this tender in the same box. The tender was missing a whole firemans front side of the tank. I forget if this was a 6464 frame or a caboose's.
Anyhow, crude, but this was all garbage . The wood, a thick vaneer,flexed under load and it was a light tender, so a weight slug firmed up the center. (landline phone or HO weight?).
JB Weld did stick here. The right batch is a near perfect color match to Lionel plastic ( the shade can vary by pigment and your mix) This whole corner is 2-3 layers of JBW molded with Saran wrap and sculpting tools, including the steps and riser to the tread.