It's an 8HP Tecumseh? So it probably has a 1" PTO? You can get an authentic Comet, but it will most likely be used. Although, there is nothing wrong with an older and/or used TC as Comet made them right and they're relatively simple. GTC makes clones of them, claims they can handle higher HP than Comet, they can't, and they've received mixed reviews. To get a real Comet TC, check Ebay, but be careful and confirm with us. Also, you can get one from your local Craigslist if you can get someone to part out a kart. Check the FS/FT section as I think someone on here is trying to trade one.
If you get a 30 series, it will be cake to assemble. It will have a backplate that bolts to the motor's face. This backplate has a jack shaft which has a small sprocket and the driven half of the TC. The driver slides onto the PTO and a bolt secures it.