Uh, how's the rest of the tractor? Particuarly the trans, as it can cost more to replace than the engine. You mention it's a lawn tractor, that usually translates to a light duty hydro which often fails before the engine which explains why there are a lot of used vertical shaft engines around. Depending on the age of the machine, you might find it difficult to get that exact "drop in" Briggs new. You may find it much cheaper to get a "generic" Briggs and mount and adapt wiring as required. I do feel that going to a local small engine shop is probably the most expensive way to get an engine. They'll order, put on their markup, charge for the shipping, etc. Here's a link to Small Engine Warehouse, I've never used them, but have seen their Ads around for quite a while.

http://www.smallenginewarehouse.com/

If I were going the replacement engine route, I'd give serious consideration to a Predator from Harbor Freight. they're a "Chi-Honda" clone and know many folks who have used them for replacements. Here's a link to a "generic" replacement.

http://www.harborfreight.com/engines-generators/gas-engines/22-hp-708cc-v-twin-vertical-shaft-gas-engine-epa-62879.html

Don't get too hung up on "HP" as, often, it's just a matter of which decal the Mfg. put on it. Don't want to get into the "spirited discussion" of what HP means in a small engine. :-) e.g. I had Briggs replace a 22HP on a GT as a warranty thing, they shipped in a 26HP(according to the decals) but it was identical in every way I could deterine. OK, I didn't take apart the carb and compare the jets, but other than some "aftercoding" on the model number identical. Also the MPL numbers were the same, including the carb, so......