You should be able to clean your stuff by dissolving in dry ethanol or/and dry methanol.
The meth will dissolve, eventually, in alcohol. Pour through a coffee filter. The stuff in the filter will be your non-MDPV bath salts, which you can then soak your tired fists in after going back to your retailer.
Let the alcohol sit loosely covered to slowly evaporate, leaving shiny clean meth. Depending on your cheapness, you might repeat the wash either on the bath salts or the recrystalized meth.
That's all speculation, I've never tried it. MgSO4 is barely soluble in alcohol, so you won't remove all trace. If you do decide to try a cleanup, please don't use HEET as your solvent. That rumor is out there, and HEET is a real thing (I'm from California). It's got methanol in it, it's probably mostly methanol, in fact, but it's also going to have flavor-crystals and new-car-scent and whatever proprietary stuff in it they aren't even required to list, probably. Stuff that might cling to your dope, and enter your blood stream, preserving your corpse to start-up even in the worst blizzard.
Finding denatured alcohol shouldn't be too hard. If you're going to be injecting directly into your bloodstream, it makes sense to take the extra time.