Two ideas Jon. First, can you use a suction cup to pull the lens forward? If so, a dab of glue might keep it there. If this is a no go, and tools come into play, Sears has a cheap strap wrench. Plumbers use them for twisting on finished chrome plated fixtures. The one (two) you want is made of plastic with a rubber strap. Put one in a vise. Put the speedo bezel in it. Use the other to clamp the body near the base of the speedo. Apply a little friction and tap the tool to separate. Rotate the speedo a third of a turn and repeat. And again. And again.
Good luck.
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