Well, define dirty and gummed up...
If you're talking extrernally, just hose it down with some motor spray. May need a couple of cans and a toothbrush to get the hard stuff out.
If the engine is seized you're going to have to take it apart. You'll also need to let us know what engine is in it. It could be the 12e if it is the original Nitro RS4 with the silver, no-countersunk chassis or the later Nitro RS4 2 with the 15fe and the purple countersunk chassis. Depending on which version you have will change what tips I can give you. The 15fe will say 15fe on the pull start case. The 12e will just say "Nitro Star". Both engines look almost identical with the only external difference being the 15fe has engine mounts built into the bottom of the case in addition to the normal engine mount tabs that are on the side of all small block nitro engines. The 12e just uses standard engine mounts.
First, NOTE: When taking apart the engines, be VERY careful. The screws most use are small and VERY easy to strip out their heads. This also goes for the screws holding the engine to the chassis. Use a screwdriver that fits the screw heads very well, press down hard to prevent the screwdriver from slipping, and turn slowly. At the first sign the screwdriver is slipping on the head, STOP! Also be sure to use the proper sized allen keys for all hex head cap screws. Most of the hardware is metric so be sure to have metric allen keys if your engine has hex head screws on it. (I'm pretty sure though the 12e and 15fe both used phillips head screws throughout...)
As for engine teardown, there are manuals on HPI's site.
The 12e is similar enough to the 15fe that you can use it's manual for the 12e teardown (they don't have one posted for the 12e). Also, the other 12s listed on their site are COMPLETELY different from the 12e so DON'T use them.
Now if the engine is locked up, you're going to need to take it apart. Here is where I'll need to know what kind you have to give you proper pointers. One thing you WILL need to know though is that you need to note how the sleeve is aligned in the engine case. The sleeve is ported and the ports need to line up with the ports in the engine case. (Except the exhaust port, you can't see the ports when the sleeve is in the engine case and the piston is at the top of it's stroke.)
Now, chances are the piston will be stuck at the top of the cylinder. To get it down, (after taking off the cyclinder head) first try pushing it down with the pull start off the engine. If that doesn't work, get a small screwdriver and place the HANDLE OF THE SCREWDRIVER on the top of the piston (DO NOT use the point of the screwdriver as it will damage the piston). GENTLY tap the tip of the screwdriver with a hammer to try and loosen the piston and get it to move down.
IF this doesn't work (and it should) try placing the engine in a small container of FRESH nitro fuel and letting it soak overnight and repeat the process.
IF it DOES work, continue to disassemble the engine. Take the piston, sleeve, case, connecting rod, and wrist pin and soak them in FRESH nitro fuel overnight. Remove them the next day, brush off any residue fuel varnish that didn't come off during the soak, dry them off, and reassemble the engine.
Depending on how used the vehicle is, this MAY or MAY NOT refresh the engine. If the engine was ran hard and has more than a couple gallons through it, it may be worn out completely.
Also, here are some manuals for the different Nitro RS4s.
Hope this helps and if you have any more questions let me know.