well...the MSG90 has the long barrel and front handguard of the PSG1 but retains the full length 20 rd mag of the G3/SG1.
I used a G&G stock, a CA SG1 cheek riser, TM front end. 650 mm inner barrel and a custom outer barrel.
It ended up looking like this.
Expensive and hard to find the correct Zeis scope and mount. I also used the CA SG1 extended recoil pad. I alternated between the PSG1 grip (not correct fr MSG90) and the OD G3 grip (correct) The MSG 90 does not have any iron sights but I did not come up with a suitable solution to fill the hole remaining where the iron sights were. The authentic MSG90 flashider/muzzlebreak) is impossible to find so I just used a constant diameter outer barrel without a flash hider.
Internally this gun was built with a reinforced 8mm gearbox, all upgraded hi torque internals and fired 470 fps with 140 spring. I did block the selector with a small set screw to shoot semi only as in the MSG90.