That 150 on your cart could make as much as 12 or more HP if properly tuned. Using a muffler for a 7 HP tractor won't help.

An engine is basically an air pump. Plug up one end of the pump and you're not going to make things better.

Here's a cheap option:

http://www.summitracing.com/parts/WLK-18394/

Or go to a local dirt bike shop and ask them if they have an old stock muffler in the scrap heap.

the one off the tractor has like a 20 hp. It is off a different engine than the 7 hp.

i have a stock exhaust if your interested it is used but same as new. it was a take off on a new buggy to put a performance one on.
this exhaust comes straight out the back by cvt cover.
you can drill the muffler out some on the stock muffler it will give you a better flow and still leave you with some bottom end power.
depending on your elevation you should be able to run a 122,125 or 128 main jet with uni-filter and drilled stock muffler,noticable gain in power.
i don't advertise it, its 30 bucks plus shipping if your interested
where are you located, should add that to your profile. helps us and dealers to know when trying to help you.

here are a couple pictures of it.
pud

I've seen you offer that muffler to every one and no one evr takes you up on it It's a good price for a muffler that fits your motor

ah thats ok chuck ,i don't have to get rid of it.i just read his posts and seen that maybe he might be on a tight budget wright now.just trying help a fellow buggyer.if i sell it i will probably find a 4 bolt valve cover engine dirt cheap and wish i had kept it

pud