The 3AJ will fit - '88-'90 twin-headlight version of yours. Benefits from better cooling and I think stronger 5th gear or at least better lubrication. Difficult to get hold of an engine without buying the whole bike though.
XT660's will not fit without fabrication work. Pre-'90 TT600s should fit, later model TT600s will not as the engine is slimmer.
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Barry
3AJ Tenere
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Barry
XT600Z 3AJ Tenere (rebuild in progress)
SRX600
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