As for the right plug rock down to your local bike shop & ask to see the NGK booklet with there plug fitment guide it will list the suitable plugs for your bike.
If it was my bike I'd chuck a piston & rings in it , and get the conrod play measured while itz apart.
Also wen waz the muffler repacked as this can cause a flat feeling with the power delivery, you can also de-coke the expansion chamber. A lot of little things add up to make a larger problem.
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