If a new RISii is out of your budget, keep your eyes on the EE. There was a black RISii selling for like 300 shipped which is decent. Another option (but not clone correct) is to look for a DD Lite rail on the EE. You can make a pretty decent clone using the Lite rail.
Now since its all said, I really think the RISii is sort of the heart and sole of the MK18. You can get away with using different receivers and BCG and such and no one would be the wiser, but the rail is a dead give away. I would try and save for the RISii. The gun dealer in mcadam NB sells the RISii in FDE for 375. Best price you will find for a new one
I have the new US Navy SOCOM RIS. Tee Hee...