Besides changing spark plugs
there are a shitload of things you can do to the little 175 twin but it's worth knowing that no matter how much you do to the little bugger, sadly it'll never be much of a racer.
Googling and eBay surfing will find the right bits like Webco cam and valve springs, Venolia make pistons for them but you may get lucky and find a set of MC brand pistons for a few bucks. It's a pity it's not a 250 or even better a 350 as they can be made to really hoot.
Off memory I think the SL175 has a 19" front wheel already so it's just a matter of knocking a bit of weight out of it, fitting fibreglass or alloy tank and guards, trials tyres front and rear and getting out there and chucking a few skids.
If it were my bike however, I'd be restoring it as they're a cool looking little bike in stock trim.
