It’s usually enough to just drop in some black dots and orbit/turbulence them around a bit. Flies are so small that it’s not worth a highres texture. If you don’t have access to orbit or turbulence, make a few crossplanes, randomly offset from the origin. Say, you make 25 of these. Slap a fly on each one and combine to one mesh. Now combine it to one mesh and emit 5 of them and rotate them randomly. Boom, 125 flies buzzing around, at the cost of 5 particles.
However, it can depend on the setting. If this is a main feature of the game or something you’ll get really close to, I’d recommend some animated meshparticles. Either vertex or scrolling baked frames. That way, they can be lit proeperly and look really cool.