agreed to the above, if you can have it off the road for a few weeks for the rebuild.. buying a 2nd hand motor esp an old one like a 4k, is really a gamble. unless you can test drive it or see it running before removal. or if your paying like $0-50 like i do for 4ac, thrash them for 6-12months then when they pop replace them..
having said that i plan to rebuild one for a bit more punt :)
1 rebuilt motor for the price of 10 used motors haha.. prob last 4 times as long tho and make an extra 40% more power than a stock one.