It all depends on what you consider to be a "proper" conversion to UMP 45 configuration. If all you want is a Stock-Block conversion using aftermarket Hera Arms parts like the one currently/recently on the EE for $6K, you can piece that together yourself. All of the parts are available through various vendors and/or stateside from HKParts using the 3rd-party importer that will give you the best deal. You can do the Barrel Swap from 16" to 8" with a hammer and a punch. One roll-pin is all it requires. Remove roll-pin, swap Barrels, reinstall pin.
A full-blown Tom Bostic-style coversion (like mine) from a USC to a UMP is an entirely different matter. Bostic turned this process into the start of a career in manufacturing HK clones, which he does to this day with the Tommy-built line of pseudo G36s and UMPs of various calibres and configurations. It is a meticulous and detailed undertaking which is not for the faint of heart, as it involves cutting off the back end of an expensive USC receiver and grafting on a genuine UMP Receiver rear section with the Stock Hinge and Push-pin Hole for the Trigger Group. This has to be done with absolute precision regardng overall length so that the take-down push-pin lines up with the holes in the Trigger Group. The conversion must be strong enough that the graft seam does not become a weak point in the receiver, the rear section of which will take some battering by the back of the Bolt if firing high energy ammo. It also has to withstand the brittleness-inducing effects of cold weather on polymer and plastic-welded polymer, especially here in the frozen North. I am pleased , to note that my Tommybuilt UMP-45 has stood up to our winter cold just fine, although my range sessions were not without some preliminary concern! Tom Bostic goes so far as to "Melt" a steel spine into the polymer top of the Receiver, thus embedding the steel across the polymer seam and lending it greater strength than plastic-welding alone. Bostic's finishing touches are also a part of the magic, with the receiver refinshed to match and even the correct UMP markings laser-engraved into the Receiver. As a result of these final details, my UMP-45 is actually registered as such by the RCMP restricted firearms database. NOTE: Despite a lengthy search, I was never able to find anyone in Canada willing to take on a Bostic-level conversion job. That may have changed in the past few years - I dunno. It is worth a thread asking, if nothing else. After all, nothing ventured, nothing gained...
As an aside, Tom Bostic is out of the conversions business, even if you had a way to order one of his creations and import it into Canada. He is all about assembly line production of complete T36 rifles and his clone UMP PCCs nowadays.
Apparently somebody thinks that a USC Stock-Block conversion (all reversible) is worth $6K. That may be quite a stretch, but whatever. We also know that a genuine UMP-45 sells for upwards of $10K in Canada, perhaps more if in unfired condition. I reckon those valuations put my actual Tom Bostic conversion right around the $8K value mark. Not bad, considering what I paid for it.
So, what exaclty is it that you are looking for? If it is something like I have, there are maybe a small handfull iin Canada. Maybe. There are definitely more genuine UMPs in Canada than there are Bostic conversions, I can guarantee you that. So, instead of dicking around with 3rd Party Importers and HK Parts.net, or chasing down an unobtanium Bostic conversion, you may as well save a few more $$ and buy a genuine UMP for $10K-ish. Its a rare HK, so you gotta pay big to play big! But that way, you are one and done, and you hav exactly what you want. Problem is that you have to pretty much be independently wealthy to collect HK here in Kanada these days. More's the pity.
Here's my Bostic conversion for reference.
