>>9371006>How hard would it be to convert a space marine into Magnus the Red?Extremely hard unless you're into resculpting ~70-95% of the toy.
His "original" form is wearing artificer armour which is totally different from the primaris marine armour. You can MAYBE adapt and sculpt over the thighs and belt and partially the chestplate. That's about it. Everything else needs to be replaced.
His "daemon prince" form is... completely different again, he's a flesh and blood giant winged demon with a couple non-marine pieces of armour. You're looking at MAYBE only being able to reuse the chest plate, heavily modified.
If you're looking for a magnus toy, there are probably many options out there as a base that would work much better despite not being an official warhammer(tm) product.