I'm sure that you will get other, more informed posts on this, but it sounds like you suffered a catastrophic failure and need to send the whole bike to me to use for parts, and I'll even pay the freight
Seriously, a seized engine is rarely too bad to correct without a lot of fuss. Get yourself a repair manual for the bike (Clymers, or another publisher) and follow the instructions. It is pretty basic. Nathan even has an owners manual for a '74 CR posted in the manual section of the site, and other than the oil line and a few other minor differences between the CR and MR, you should be able to follow the steps for servicing and decarbonizing the engine in the owners manual, and use that as a guide toward fixing your bike, but a manual is the better way to go and money well spent. Once you pull the head and jug, take a close look at the piston, if the skirt isn't too badly scored, you may be able to rub it out with oil and very fine emery paper, or sometimes jewelers rouge will work better. If the cylinder isn't too badly scored, sometimes just breaking the glaze with a hone (a 3 stone hone that can be purchased at most motorcycle shops or auto parts dealers) can clean it up pretty good. If the scoring is severe, you may need to get a new piston and rings and have the cylinder bored out to the next oversize. There are quite a few resources listed on the site, and Wiseco piston sets that include the piston, rings, wrist pin, and clips, are available, as are the top end gasket kits (cylinder base and head gasket). The cylinder can be bored by a local bike or machine shop for not a lot of money. If the bike is a 76 and you don't know when the last time the top end was freshened, a new piston and rings may be the best way to go, to begin with. If you don't have an inside micrometer, take the cylinder to a bike or machine shop and ask them to measure the inside diameter in a couple of places to make sure what your bore size is, or if you are out of tolerance and will need to re bore anyway. I don't have the specs in front of me, but I believe that your bike had a 70 mm original bore. If the inside measurements exceed 70.10 mm, it is time to go to the next size over, and the piston and rings you get will need to be for that size.
If you can't get the jug off, and the piston is froze in tight, I use the very unscientific method of spraying the inside of the cylinder with a good penetrating oil, then using a piece of wooden 2X2 against the piston and moderately tapping the wood with a hammer until the piston breaks free and you can remove the cylinder. Most of the repair manuals you can get for your bike will describe how to make your own device to support the piston when you go to put the jug back on. Essentially it is taking about a 3/4 inch piece of wood about 3 inches wide and about 7 inches long and drilling a 1 inch hole about 4 1/2 inches from one end, then using a saber or jig saw to make two cuts from the end to the hole. This device can then be slid under the piston and will fit around the connecting rod and support the piston when you go to put the cylinder back on.
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