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We did this with our first 31H (actually a 31W). It's actually more cost effective if you're patient and good at sniping on eBay. We got the empty cab (great shape) from a guy who had tons of stuff. All that was in it was the lower baffle. My dad then bought the upper horn section from the same guy (horn, bearing, driver, 2-speed motor) for 60 or 80 bucks plus a free 15" speaker from some wrecked Conn, built the upper shelf, got a 2-speeder on eBay for maybe 25 (including shipping), a crossover on eBay (maybe 12?), and found an awesome deal on a 21H amp (110 including shipping). He built the lower motor mount and upper platform from scrap wood, so it didn't cost a dime, and then we had the amp converted to a non-FC type and I wired in the speakers and motors. All in all, it was cheaper than most 31Hs we've seen, and it has 2-speed capabilities. The thing sounds damn good, looks better than most 31s we've seen, and feels great since we got it as a big empty cab.
So, eBay and time will be your best friends here. You'll need to be patient enough to wait auctions out; if the prices goes too high, don't chase it. There's always more of this stuff out there. Some bastard rips a Leslie apart everyday. I hate to encourage that behavior by buying the parts from him, but if it means I can resurrect a different dead Leslie it's OK by me.
As for a parts list, here are the big items:
- Amplifier. You can get a non-Leslie amplifier, many will say they sound just fine, but just as many (or more!) will say the only real sound comes from a real Leslie amp. Keep an eye out for one-speed models (45/21/31/etc) because these often seem to go for lower prices and can very easily be changed to two-speed in a variety of ways. If you can solder, all the better. Be aware that some models require a Field Coil speaker, if you don't have the cash for one of those you'll need to replace it with the proper high-power resistor. That'll add a few bucks, but nothing too bad, and I believe it should work just fine.
- Crossover. Very easy to find, I don't think we've paid more than 20 bucks for a crossover including shipping. Oftentimes they need a recap job, but that's just a few more bucks, and usually it works alright until you get around to doing that.
- Horn section. This is the big one. You need the horn, the bearing assembly, and the driver. The driver can be different models if you have trouble finding a good V21 (original driver), but there's usually an auction or two with the complete upper setup. This will probably set you back, because the bearing assembly is pretty costly.
- Lower speaker. Usually a 15". If you get an older amp, you'll need a Field Coil speaker, which will set you back quite a bit! There are several popular substitutes out there, sometimes you can find a good deal on Leslie 15" speakers. I suppose if you want the original sound, that's what you should go with. But as my tech friend says "they only used em because they could get em cheap!" Search around and you'll find lots of suggestions on different lower-priced speakers to go with.
- Lower baffle. This one I don't know much about, since all of our Leslies came with theirs intact. I'm sure there's a few on eBay, you'll also need all the mounting hardware for it, and you'll need to make sure it'll fit your cabinet (don't know if there are different versions). A few places sell these new as exact replicas, but that'll set you back a bit more. You'll also probably want to get new bearings for them, all of our old Leslies rumbled in the low end because the old bearings allowed that thing to rattle around. Again, not too much added on, I think they're 15 bucks from tonewheelgeneral.com
- Motors. If you go one speed, you'll have no trouble finding cheap motors. If you go two-speed, you may have to wait, but you shouldn't have to spend a fortune on a pair of 2-speeders.
Did I miss anything?
Hope that helps!