Weiger, Bulgarian and other magazines were converted to Galil use, not Valmet. Do NOT use any 5.56 Galil or modified magazines in a Valmet.
IIRC Investment Grade firearms once posted a Valmet rifle modified with a fed ramp or magazine adapter held in place with small tig welds. Comments made it clear few saw this as an upgrade or improvement.
Old pictures used to show an expanded receiver with a split front trunnion. It may happen that this does not scare you, contemplate trying to replace that Valmet trunnion. If the bullet is set back into the case, because it struck the barrel face when being chambered, this is merely the second error of a series which will lead to catastrophic failure. The first error is using the wrong magazine. The third error is continuing to use the wrong magazine because it has not maimed you or destroyed your rifle, yet.
Yes, Valmet's are expensive. So... should not one value the rifle enough to use the correct magazine? Imagine how much more costly it would be if spare parts and magazine were cheap and available.
There is an exception to what I have posted. Those magazines are not any less expensive or more available than factory original. Just buy the factory original.