My attempted solution for the gas tank mod on a '05 CR250R conversion has been to cut the bottom of the tank off and weld a new plastic bottom on. I bought a piece of 12x12" black HDPE from Graingers. I cut about an 1 1/2" off the bottom of the tank, welded on the new bottom, and welded the spigot/sump well off of the old bottom onto the new bottom. I used a propane torch which is definitely not the right tool, but I got results--they're not pretty, but I couldn't guarantee that I would have made pretty welds with a hot air plastic welder anyway.
I think the tank will hold a little better than a gallon, but haven't measured. The bike will be a moto only bike, so the capacity is not a problem for this vet rider.
The pictures show that there is plenty of plug clearance, even while the engine is sitting about 3/4" higher than it will after I weld up the new frame rails. I believe I'm going to be able to use the stock unmodded head and radiator hoses too, which will be a big plus.
I have pressure tested the tank since taking the pictures yesterday, and one section of weld failed, so I'm back to the torch and keeping my fingers crossed that I'll get a good structurally sound weld all the way around this time.
