The engine is a Robin 295. If you order from a golf cart parts supplier you'll only need to give them the year and model of the cart, 1993 EZGO Marathon. So if you're looking on golf cart sites look for 1993 Marathon parts.
You can get a enclosure for a Marathon and a propane heater that fits in the cup holder. You'll need a Marathon roof to add a enclosure.
Yes you can mount a bed/cargo box on the back. There's aftermarket beds on all the golf cart sites. If you can't fine one for a Marathon give Allsports a call and they should be able to take care of you on a rear cargo box.
Take a look at the banner ads on the lower right of the main forum page or click the Sponsors tab on the top of the forum and look through their websites for your parts and accessories.
Here's a link to an EZGO 295 service manual in our EZGO Resource section of the forum
EZGO 295 and 350 Engine Service Manual
Don't worry about asking questions that's what we're here for.