You can get a longer handle to gear them up. What gear ratio are you looking for? 6.3 and 6.4 to 1 are the most common, all purpose ratios.
If you're really wanting to change the gear components I'm sure one of the reel guys on here can do it, provided they can find the parts. Anything you found would be aftermarket for sure. . .unless it happens that the Mettle shares the same components as some other major brand name reel.