Exhaust mod for the 3rd gen

My backbox is rusted to bits and was crumbling away, so i had it cut off (because of the rotten bolts that wouldn't budge), so now the car sounds a lot cooler but a bit annoying at the same time. The mechanic suggested getting a muffler from a bike, like a 1300 or something of the sort, but i don't want to spend a lot of money and was looking for the diameter of the pipes of a bike muffler. My jimny should have 40mm exhaust pipes, would a 51mm muffler be as big as a 1300's?

If you can make if fit I reckon a bike silencer would work. In the UK you can get a "cherry Bomb" for a car, again if you can weld and make if fit it would work.
I have a straight through rear silencer, but I had it custom made by an exhaust company. Not 100% sure what you are asking about the sizes, but my exhaust silencer uses a larger diameter pipe than the standard Jimny Exhaust.
 
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There's a chap called Tim Lambert that makes up a bolt on "sports" exhaust for the Jimny. You should be able to find them for sale on ebay. He's a regular on the Jimny Facebook groups.

I was thinking about using a 51 mm exhaust, and thinking about it should be better flowing than stock, I don't wanna spend much so I think I'll just go with something from Amazon.
Also, would a cone filter get rid of the whistle that the stock filter generates at >3000 rpms and moderate speeds? I would like to make it sound a bit like an old rally car if it's possible