Need engine interchange info 750 Seca to Maxim

I bought an engine supposedly for a 83 XJ Maxim. The shifter is in the wrong place and it turns out it is apparently from a Seca. By using the shifter that is on the engine it will be on the wrong place for the Maxim. Everything else looks to be the same.
From what I see, I should be able to remove the shifter and use the Maxim shifter which pivots from the foot rest instead of from the side of the transmission cover.
When I first checked the compression it only had 10-15 all the way across. I shot a couple squirts of oil in each cylinder and it jumped to 155-170 all the way across. I checked it again the next day and it is 55-65 all the way across, even with a couple shots of oil. I figure that I knocked some carbon or dirt loose that probably got under the valves. I don't know how many years the engine sat or the miles. I only gave $50 for it but the owner told me that he had it running.
I am going to check the valve clearances while the engine is out. I thought about installing a set of rings and gaskets in the top end. I know nothing about the transmission. I don't know if the XJ transmissions had a problem with the gears or not. I will also pull the cover from the transmission and take a peek.
I know the transmission is good in the engine on the bike now but it has one cylinder that I can put my finger over the spark plug hole while running and I feel nothing. It also blows oil from the breather after running for several miles. I suspect a hole in the piston or a chunk missing in the side of the piston and rings. It runs most of the time on two cylinders and then finally kicks in on the third. I actually got the dead cylinder to start hitting once but then it quite firing just as suddenly as it started.
I was experimenting with switching the coils and wires around when it started firing and I lost two other cylinders. I have replace all the coils, wires, rebuilt the carbs and had the apart 4 other times, used a home-made YICS tool to set the pilots and about every thing I can think of. Still, the one cylinder has no compression at all.
I actually bought this Seca engine for parts in case I needed maybe a piston and possibly a set of jugs. The bike belongs to my friend but he really loves it. Any info on the engine interchange would be appreciated. The Haynes manual doesn't say anything about interchanging the engines.

 

Swapping the shifter assemblies (including the cover) should be all you need to do to pop that thing in. Make sure you use the Maxim motor mounts, the Seca's are a bit different. Definitely hang on to the old mill, the lower ends are completely compatible. A top end kit and you'll be back in business.

 

Thanks Robert,

When I first bought the engine I was very satisfied that I had a good deal. Then I saw the shifter in a different location and began freaking out (kind-of) thinking that I spent money on something that was not compatible.
It concerned me that when I first checked the compression that it really jumped up to decent numbers and then dropped back down. I may even install the new rings and gaskets in this engine before putting it in. I was hoping to just switch engines and worry about the top end rebuild later.
Can you tell me if the XJs typically have issues with the transmissions shifting or coming out of gear? Some of the old Honda V65 Magnas had issues but that was because they run like scalded dogs and their owners abused them. I have three of them. Extremely quick!
In spite of it all, it is a good bit of work to change an engine and I would prefer to know about any possible transmission issues beforehand.

Thanks,

 

The only transmission issues I am aware of is the potential for the starter clutch chain guide/tensioner to break into little pieces and rain down on the tranny, locking it up. Other than that, there is the possibility of a certain transmission drive dog having it's edges worn away by a careless prior owner which will cause poor engagement, neccessitating the undercutting of the meshing dogs. Which part it is escapes me but I'm sure someone will jump in and clearify that for us.