A repair method
I did the same thing to a KLR 650 clutch basket - it does make you feel stupid.
If you decide to use it as is I would not worry about balance - there are much more out of balance items thrashing around in the engine (think balance shafts, cranks) so it won't have much effect on vibration levels and the level of imbalance will be so low as not to affect bearing life. If I had to I would try it without the one post I think it is likely to work OK but you might get a bit of tilt in the clutch plates which would cause drag- I certainly would not break another off to try and balance it which will just weaken the whole clutch further.
Now the purists won't like the following, but I have a frugal nature and don't like replacing things unnecessarily so in my case I looked at my repair options. liked you I thought of welding but the access was awful and maintaing the alignment during repair was going to be awkward and any weld achieved was going to be very questionable quality. So in the end I did a mechanical repair as follows:
- The leg on my basket broke off half to two thirds way down its length at the bottom of the thread tapping in the drilling down the centre of the post.
- On the basket I contine the hole drilling through to the back of the basket (ie changed it from a blind hole to a through hole). Hole size was 6mm to provide a clearance hole for an M6 bolt.
- From the back of the basket I then countersunk this hole before fitting an M6 panhead bolt cut to length so it engaged the threads of the broken part. Fitting the bolt then secured everything back together.
Things to remember if you do this are (i) the bolt needs to be a countersunk type or it will catch behind the basket (there is usually little clearance) (ii) it only needs to engage into the broken part of the by about 1 bolt diameter -say 6mm for it to be structurally effective and (iii) most importantly it needs to be well secured so it will not come undone - I used both thread locker and centre popped the periphery of the bolt head. You may also need to shorten the clutch bolt that goes into the top of the post slightly so it will tighten up fully before its head reaches the new bolt coming from the other direction (in my case I didn't need to do this).
I had a pillar drill do do the drilling but with a bit of care you should be able to do all this with a handdrill (and you cant do much more harm than where your at).
My repair has done 18 months and 25,000k (including some quite brutal dirt work) without any problems.
Anyway best of luck if you decide to try a repair.