You said you think the axle is bent? Then I wouldn't worry about trying to find Murray parts, especially if they are hard to find anyways. And I wouldn't look for a jackshaft plate nor try to make one. Your output shaft on your engine is going to be on the driver’s side, your getting a new axle, so just plan on moving the axle sprocket to the driver’s side, it’s so much easier.
I’m rebuilding a Manco kart, don’t know the Murray karts as they may be slightly different, but I’d approach your build as following…
First I would try to plan out want you want before buying new parts. I have found that a lot of your part choices will effect other part decisions. You’ll in up with parts that don’t work together or end results that you don’t want, like gear ratio/speed, etc. Engine choice (size of the output shaft), Centrifugal clutch or Torque Converter (tooth count on their sprocket), axle sprocket (size/tooth count), axle brake, wheel size, and determine the final gear ratio/speed that you want.
Disassemble your rear axle. Measure the axle’s length, diameter, and its keyway. Length is the main measurement; you might end up going with a different size in diameter/keyway depending on the other parts you decided on. 1” axles and ¼” keyways seem to be the norm; there are other sizes but again…plan ahead with your other parts. You’ll want an axle with a keyway length on the driver’s side, if not the full length of axle. You’ll want to measure the wheel bearings on the rims/wheel hubs and get an axle that has a step down with that diameter, length, and keyway size that will fit the rims/hubs. Put in new axle bearings that match your new axle. Not sure what Murray has or how they are mounted to the frame. They make new axle bearing kits with new brackets that you could weld to the frame if fitting new bearings is an issue.
Not sure how the brake caliper mounts to the frame on a Murray or if you have those parts, don’t see them on your images - just the brake disc. So you may have some fabrication there…
Other’s can chime in if I missed anything… I’m no expert when it comes to karts.
I’m in your area (Redington Beach) next week, hope the sun is still shining when I get there, I could use some rays!