Generc Question About Cost Of Engine Replacement

I have a vintage Mack firetruck powered by a gasoline Thermodyne 6 cylinder dual system that in addition to being almost 50 years old, has 24000 miles on it. I just bought it and what i hoped would be a quick fix to a power and backfiring problem is getting expensive. From what I read, a re-build would NOT be the best answer as I would have to many other components that are not replaced and ready for failure.
My question - just a ball park figure - is what would I find to be the cost of replacing the engine? Anyone here replace one of their big rig engines and can give me an idea? I know there are going to be differences depending on what engine goes in (gas or deisel) and what size, but I'm not looking for specifics. 10k? 15k? 20k?
I need to know where to draw the line on this "toy" before it becomes to much of a money pit.

 

thanks for the replies - I've been a bit busy. I've already put $2700 into a full bumper to bumper inspection and maintenance of all the basics (hoses, belts, ignition system (new points, plugs ,dist), oil change, coolant flush...). That was supposed to include trouble shooting and repair of the power problem and they did work on the carb and gave me the keys. Drove off and hit the first hill and lost power, had backfiring, AHHGGG! Went back and the Mack shop said it probably needs a carb rebuild and sorry, we dont do them. Took it to SE Deisel (Chattanooga) and they will look at it for me but are dragging their feet - 3 weeks and they are still "estimating" what a full timing inspection may cost (yes, Mack replaced all the points, plugs etc, but did not check the timing - never going to Mack again).
I've since found that a carb rebuild should be under $500 but still have no idea about any repair to timing components. I had bought after being told by the department who ran it, that it was in "great shape", but then it sat for a year and it aint so great right now, LOL! Still lookin for an old time Mack afficianado in the SE who I could hire as a mechanic so I don't have to rely on a deisel shop to repair my gas engine. oh well! will try to attach a couple of pix and yes ol Bullet, I am a member of SPAMFAA and am also on the bigmacktrucks forum, thanks for mentioning that though, I will likely end up trying to find another local collector thru the site (most seem to be up in the north east).

 

Heavyd - no, it's not a cabover, its a midengine - the 'doghouse' between the rear facing jumpseats is the access to the engine. I'm not sure if removal would require lifting off the roof of the open cab and picking it up, or raising the vehicle and dropping it.
not sure if making it horse drawn would eliminate the amount of s*** I have to put up with to get it in a parade though!