There's tons of info on conversions over on the giant scale engines page. I'll give you a brief rundown of what i know, though.
The three things you need to do a conversion is a prop hub, a muffler, and some sort of way to mount the engine.

Here's a link to how to build a really cheap muffler, that works great.
http://www.rcfaq.com/answers/engines/mufflerplans.htm

The engine mount is kinda one of those do what works kinda deals. Basicly, as long as its good and rigid, and won't come loose in flight, it'll work.

As far as what kind to convert, theres a bunch of them. Homelite 25cc blowers, ryobi 31 cc blowers or weedies, mccollough 32cc, homelite 33 cc, homelite 45cc, poulan 46 cc, among about a thousand aothers.
Chances are, you've got one laying around somewhere that'd work.
I picked up a 31cc ryobi weedeater engine from Ebay, for 35.00, it was complete minus the shaft, and it made a great engine. I bought a new 45cc homey chainsaw, and its a real stump puller. Gotta love swinging 20" props.
The long and short of it is to experiment, and poke around and see what you can come up with.
For 30cc size engines, i'd keep the weight to about 14 or 15 lbs max, 12 is much better. With the 46cc, i'd think you could go 20 lbs with no problems.