I have a blown supercharged 3.8 gtp. Can I replace it with a 3.8 oem and use my supercharged parts

No. You need to find a supercharged engine if at all possible. The parts will fit, but there are some internal differences that are specific to the supercharged model.

If your S/C engine has rod knock but not missing, YES, you can go a few ways to replace the short block with N/A(non supercharged) bottom end but it requires getting educated in modification and retuning. So you can bolt your top end on any series 2 or 3 3800. Paying attention to oil pan, balancer, etc. Because its higher compression it will require exhaust upgrades ie headers/performance exh manifold. ECM retune to adjust timing curve and fuel control parameters (basically the tuner will reduce timing then replace the fuel control part of programming with supercharged fuel map), Look for Intense Racing, Overkill, ZZP and others with solid reputations in 3800 tuning. You can mail them a new/reman computer, your VIN and mods done and they can adjust accordingly. The head, lower intake, blower, balancer and brackets need swapped to other engine. It does result in a bit more power if properly tuned. Also remember doing this eliminates low octane fuel ever. Its doable, many do a "top swap" on N/A cars but have to modify and retune accordingly. For easy drop in just look for another L67/L32. Series3 only have throttle by wire and would require adapter or mods but from 05 up has newer "GEN V" blower intake and few other upgrades, but cost more. If it says Series 2 or 3 you can use it. Differences between bottom end are rods and pistons. Rods are different length and pistons are built to lower compression for supercharger. Google Grand Prix, Regal, Bonneville forums and do some searches. Good Luck