XJ engine identification

I recently bought an engine off ebay that was supposed to be from an 83 XJ 750 Maxim. I finally got it home and was looking at it. It is complete except for the carbs. I checked the compression and it appears to be withing limits since it has sat for several years.
It looked somewhat strange though. The shifter on the 83 Maxim pivots from the foot peg whereas this one pivots from the side of the plate on the side of the transmission and the shifter points to the rear rather than the front.
I am not sure what this engines is now. All the mounting points appear to be in the same location. The engine is dry and appears to not have any leaks.
If I install the engine the shifter will be too far back, or too close to the foot peg. Not being very familiar with this bike, I need a bit of help identifying the engine. The engine currently on the bike has no compression on one cylinder and runs like crap. The bike owner thinks it runs good.
I bought a set of rings and gaskets for the old engine but figured I would try to change the engine and then rebuild the old one when I had time. I rebuilt the carbs, checked and rechecked them several times but it is practically impossible to set the carbs with a dead cylinder. Kind of like wetting into the wind, it keeps coming back into your face.
Because of the engine having absolutely no compression on the one cylinder, I am concerned about what I am going to find once I get into it. Could be a hole in the piston, a ruined cylinder wall and who knows what. After running the bike for a while it begins to puke oil out the air filter which means a severe case of blow-by. I was in hopes that this engine would buy me some time until I could get a chance to tear his engine down.
Unless I can change the side plate on the transmission so I can use the other shifter mechanism I may not be able to use this engine.
The guy I bought it from told me it was from an 83 XJ750 Maxim. It has been in a storage facility for several months now so I either use it or sell it.
I can look up the numbers on the engine tomorrow. I just happened to think someone on here might know what I have. I did see something on the engine that said 2500cc, or something. I know it was 2500. I feel this will be a good engine but since the bike is not mine and I am doing it for a favor I hate to get into more than I need. If anyone has any information on what I have said I would appreciate hearing it.
Thanks

 

Your .sig doesn't list which bike you have...

You're probably thinking of the oil capacity, which is stamped near the oil filler hole on the right side. I'd have to go look at my 82 to figure out where any identifying marks might be, but I'm reasonably certain the engine size is stamped on the front of the engine somewhere, in the middle.

In your case, I'm wondering if it might be possible to just transplant the top -- cylinders, head, pistons and rings -- and leave the bottom end intact. That would leave your shifter assembly in place. Someone who's actually done this might have a better idea, however.

 

The VIN# from the motor will tell all, it should be stamped on the right side, on top of the crankcase.

 

The engine number is 562-150796. I don't know where to look it up at to determine what size the engine is.

 

That's a G not a "6." Your prefix is 5G2, which means the motor is from a 1981-83 USA model Seca 750 (which explains the shifter setup.)

VIN decoding here: http://xjbikes.com/Forums/viewtopic/t=14577.html

The 5G2-150796 is what we call the "shorty VIN" and it's all that's really needed to identify the bike. (Actually the "5G2" is all that's needed.)

Model-specific "engine transplant" info is available in Len's new "Information Overload" thread: http://xjbikes.com/Forums/viewtopic/t=27544.html

 

I appreciate all the info. I spent about half an hour looking for the size of the engine and finally found it. It is located dead center and right below the front of the head under the exhaust ports. It is marked 748cm. It makes me feel much better knowing it is a 750 and not a 650.
Now for the problem. When I first checked the compression I was getting from 15 to 25 all the way across. I attributed this to the engine sitting for several years. I did a wet check on it and it jumped to around 120 - 165 across. When I first squirted the oil in the plug holes the number one cylinder shot up to 200. I performed another compression check this morning and got 35 to 45 all the way across. I shot more oil in the cylinders and it only changed about 45 to 55.
There must have been some dirt in the combustion chambers that got under the valves. Now I am afraid of it, too much work to install it with a problem. I have a set of rings and gaskets, so I plan to rebuild the top end and install it.
This forum is a great source of information and I certainly appreciate it.