Yeah,bottom ends are the make or break on these bikes,they cost a fortune to put bearings into the bottom end,but to offset that the crank would last 3 times as long as any other bike - you just gotta have a good one to start with.That's why that XS that went on TradeMe a few months ago was so tempting...it was a rat,but with a new bottom end.
A mate of mine has a pile of XS 650s - one for each purpose and enough parts to keep building them for years.He has a couple of 750s - one is a flattracker,used when we used to flattrack at Rosebank...the other is in his post classic racer,the motors are standard otherwise I think.One I think he used the old 750 twin pistons in - gave him a low compression ratio which softens out the bottom end punch.Otherwise he always uses the GT750 waterbucket carbs on them,just the right size and easy to set up.He got my carbs working just right this year after a couple of attempts - I also have some Blue Magnums (a few actualy) which the dude from the Graveyard has all the bits left.I have run these and will do again when the carbs on it finally throw in the towel...they are a bit hard to set up and need constant readjustment,like several times just on a ride! You don't want them...the blue will look silly on your bike.