If you can get it cheap, buy it. Only the bean counters will care that it is not factory original. The 750's are very overbuilt and fun to ride but are not very fast bikes except for the 750SFC.
The big question, is the condition of the motor as the crankshaft can get expensive to fix. But, both my 750's (1968 and 1969) are still on the original crank and pistons so you may be OK.
And after looking more at the picture, I think you may have a 1970 750GT with the wrong fuel tank. There are many on the forum who know more than me and can answer the question of what you are looking at.