Is the leak in a spot that is visible? on a weld?
There was a product called "metal weld" I think that might do the job but I would think it would be a temporary fix? And if it was visible it might look pretty ordinairy anyway.
Try Choppaweeza for a second hand tank maybe?
THe metal weld/liquid metal was something we used to carry in the bush to fix dirtbikes as a temp fix, might be worth a look till ya fix it
the oil tank is more than need a very good clean-out if not clean inside in the last couple of years as they get a biuld up of sluge in then from condensation , waste products from used oil etc , any dry sump engine should have there oil tank clean out every so often , some are that bad i have to put nut's & bolts with kero & shake & roll it around to have inside cleaned , i would pull it out & repair her proper then a very good flush out would not hurt her @ all, cost = next too nothing
In BHP years ago, we used a product called Belzona. I used it on everything from leaking tanks to rebuilding shafts under bearings. It's good stuff. Try blackwoods for the supplier. Hope this helps. Hutch.
http://www.belzona.com/products/1000/1111.aspx