The ASP/Magnum FS-52's exhaust hardware should be compatible...
The PIPE Here...I've got an Enya 46-4C AND 60-4C myself, and I've seen the exhaust port threading as being spec'ed as an M10 x 0.75mm thread, the same as used on the OS FS-40, -48 and -52 mills, and on the ASP/Magnum FS-52 four-stroker mill
The ONLY engine maker that uses 1 mm thread pitch on the majority of their mills IS Saito, which only uses a 0.75mm thread pitch on their smallest FA-30S and FA-40a mills. Everyone else seems to stick to the 0.75mm thread pitch for their mills, so far as I know. I've also requested RCGroups to make a "sticky" thread for just such needs
, and some responses have agreed with that idea.
The ORIGINAL Enya engine manual for both the 35-4C and 40-4C are available here
as an Adobe Reader PDF format file, in case you might not have one - and if you contact W. Bill Jensen at his engine repair website
, it's quite likely HE could get you the original part, or one that can fit your mill, as he's now become the North American distributor for Enya four-stroke mills and their parts.
Hope these linked lines of text can help get you a muffler for your 40-4C mill...