And wouldn't it be nice to find that Kemtron had made some?
Kemtron was a small producer...they did not have the equipment necessary to produce die cast locomotives. Also, the production run would have had to be substantial...for they would have needed to pay for all the tooling and the equipment. Further, Kemtron's quality standards are far higher than that boiler and tender. It is essentially the same as comparing my MDC 0-6-0 to a Pacific Fast Mail 0-6-0...it isn't fair to either as they are apples and oranges.
If you look around, you'll notice that different suppliers will have a certain medium...and that's pretty much all they use. Grandt Line is plastics with very few metal castings. Precision Scale is brass but with a growing line of styrene (and you don't see styrene in their older listings). Mantua was metal with a small bit of plastic & such for insulation. Later on they produced some plastic boilers...same thing with MDC.
Resin manufacturers do very little with styrene and brass. Frequently, the styrene castings in kits are not produced by the same people as the rest of the kit. Cliff Grandt frequently would make the tooling and cast the styrene parts for other people such as Don Winter. Styrene requires the greatest cost to start...massive amounts of money for tooling and expensive machines. You can't purchase a machine for a single part, typically.
Casting metal varies. The boiler like yours would probably require a spincaster, a metal furnace, and some tooling. It may have been made with vulcanized molds. This is not as cheap as you might think...there is a reason why Bowser would cut and lengthen their 4-6-6-4 boiler to produce their 4-8-8-4 and not just produce a new mold. On the other hand, some of the small order white metal parts in craftsman kits are produced with low temp melting white metal...and some non-American manufacturers (Railmaster comes to mind) use this to produce super detailed craftsman kits...you can use resin (rubber) molds for this.
Laser cut kits typically just require a laser cutter and someone to produce the detail parts. The laser cutter is the cost here (and probably a computer to control it)
Lost wax casting requires either tooling or nice masters. A mold is made of the master, and then a wax duplicate is produced in this new mold. A plaster mold is then produced around the wax and the wax is vaporized. Brass (or another metal) is poured into the mold and the mold is broken to retrieve the part. It is suitable for low runs...if you have an adequate furnace to melt the brass...1700+ degrees F. Spin casting can be used here as well.
Resin is standard for short run rollingstock. Much of the cost goes into the molds and the material. This is the easiest to do at home...you can get started for $50ish. You need high quality masters and quite a bit of labor on the casting. (I've done this) For professional castings, you pretty much need a pressure pot and possibly even a spincaster (why I don't currently do this). Typically 20-70 castings in the life of a mold.
Brass is the most labor intensive...but quite suitable for even lower runs than resin...1. That is why it is expensive even with cheap chinese labor. Sheet brass is the basic material here. It is cut either with machine tools (or by hand at home). A miniature machine shop is used here...but with solder instead of welding. The castings are produced with the lost wax technique. Photo etching is commonly used here...especially in kits for home use...it is a labor savor that requires additional equipment. The results of photo etching are much the same as laser cutting...but the production is completely different...acid baths, uv lamps, and such.
Limited run diecast is a recent phenomenon. It is directly tied to the low labor rates of foreign countries with, essentially, third world standards of living but 2nd or 1st world education.
It is very expensive to add a different product line to you factory unless you have to do so. That is why I cannot think of a single manufacturer to use more than 3 of the above techniques. The boiler is not Kemtron.
The kemtron draw bar is a replacement part. Odds are good that it is plastic to isolate the tender from the locomotive...as the pickup on diecast locomotives commonly caused opposite polarities on the tender and locomotive (one picked up the left rail, the other the right, and a single wire connected the two). My brass 4-6-4s will short out if the cab touches the tender...and some of (or all of) my diecast locomotives are the same. For Kemtron to have had a bunch of a simple styrene casting made is one thing...especially a simple casting...having boilers made is a completely different animal.
A different manufacturer may have made the locomotive...but it was not Kemtron. You can tell the artist by the stroke of the brush...and it is neither the right stroke, the right subject, nor the right brush to be Kemtron. Now if it was a narrow gauge 2-8-2...I'd question it for a second or two...