1973 Z1- any known engine weaknesses?

I recently purchased a 73 z1 off of ebay. The bike is no where near original condition and isn't going to be after I'm done with it. A lot of the rare and hard to find parts are gone, and as I've been taking it appart I keep on finding kz parts on it. Which doesn't really bother me because then I won't feel bad about riding it when it's finished. Anyway, I have the engine completely torn down and the cases split. It is deffinetely a 73 z1 case, supposedly the original engine, but the parts inside, I don't know. I can tell the cases were split at one time. The head is a 900 with stock sized valves and stock cams for now anyway and, I'm going to add ape springs. I'm putting on a 10.5:1 1075 top end, and 29mm smoothbores. Some of the trans gears look kind of rough, and they aren't back cut. I thought I read somewhere the z1's had trans issues. Is this true? Is there anything that should be upgraded that is notorious for failure in the bottom end? Will a kz1000 trans fit if I replace the one that's in it? I'm new to the kz/z1's so any help would be appreciated.
thanks

All the parts will interchange between the Z1 and KZ motors...except for certain MK11 parts. Some of the early Z1's were notorious for breaking 2nd gear adn that's why they started making the hardened gears. Or just replace with a KZ gear. Many think the Z1 trans is weak because of the way gears are made. Z1 gears form a V at bottom of gear which is prime spot for cracks to form while a KZ gear form more of a U at bottom of gear which makes them stronger. I know a guy who uses stock Z1 trans in his 9 sec. dragbikes and he doesn't have a problem...probably back cut but the gears themselves hold up. Z1 trans should be good for the street but if you have issues with trusting them then drop a KZ1000 trans in there. Just for your information, a MK11 trans is supposed to have a .030 thicker 2nd gear so they would be best trans. One guy I know was using a back-cut MK11 trans in his dragbike running 8.70's so the strength shouldn't be an issue. If you have the cases split and can afford it, I'd go ahead and have the trans back-cut. Shouldn't be necessary on a street bike fut woul be more an insurance issue. I would have the crank welded also. Not really necessary on a street bike but, again, an insurance issue. You are already in there so this would just make your engine stronger...in case! Everyday rider that isn't abused could get by with a stock crank and trans though so it's more an individual judgement issue.

thanks alot for the info, very helpful. Is there anything different about the shift forks/ fork drums between z1's and KZ's? I probably won't get the crank welded because it will only be on the street and if I would ever race it I wouldn't use a slick, which I was told that's when you need to get the crank welded. It isn't getting a turbo or anything, possibly 33mm smoothbores, cams and a ported head.
thanks

No difference on the forks or drum.

I've broken second gears and a crank centerpin on the street with a small turbo. after that I went to MKII and GPZ internals. It keep running on half an engine!

Well, the crank should at least be gone over to make sure it's true. I'm sure that your eye won't catch a bent rod and although it would work why not get it done right? I had my crank sent out and done and two of the rods were bent. They sell the billet gears and other heavy duty stuff. Why not go and build the bottom rock solid? Never know when that fever will hit and then you will be glad you did the bottom.
I had a 1075 kit in mine and just did the top end, lasted a season. Timing chain stretched from the .410 cams and caused metal flakes to come off the block plugging up the oil pump and destroyed my top end. I should have split the cases and put a heavy duty chain and rollers from the start. In retrospect it would have been better to just replace head gasket when it blew instead of 1075 and cams and avoided all of those other issues. But if you are going to build it build it right. If not build it stock and avoid the sure headaches that will come later. Have a fast bike and your prize bike. Just saying...