It's a Poulan, basically the same as all the 18/21/24cc variants just a little bigger. I don't think I have any spare old style two piece mufflers left that aren't slated to engines or I'd offer one of my mufflers. Skip the backplate mount it just adds extra weight, instead get some long 10-24 screws that are threaded all the way or some 10-24 allthread and thread it into the back plate holes, thread on some nuts to hold the back plate on, will probably take two or three to take up the space needed for the curved back plate then put fender washers on both sides of the firewall of the plane and nyloc nuts on the inside of the plane, works great, I used this method on a 40 size stick so you can't say it's impossible... For the prop hub it should be 5/16-24 threads, only the long shaft engines are odd and I think they're just 3/8-24
There's a pic of some 21cc engines I put together, take note of how far I trimmed the coil mount, that is far as I would go, no further, you can see the mufflers I use as bases, that style is MUCH easier to deal with. You'll also notice I used the stud method for the carburetors too.