I have a craftsman snowblower model 536885020 and it won't start. I replaced the spark plug but I think it's not getting gas.

You can check for spark by taking the plug out and connecting the spark plug lead. Hold it against the block (non painted area) with something insulated (wood) and try to crank. Look for spark.

Put the plug back in and crank the engine a number of times and pull the eplug out and see if it's wet.

Get some carb cleaner and spray into spark plug hole. Snowblower should sputter at least once.

If all the baove works, then, suspect a gas problem.

If it's a gas problem, it's probably due to not using a fuel stabilizer

Come back with the results.

Hi,
After a bit of research I would that the owners manual is available and a service manual. Your engine is a 143.816012 which is a Tecumseh HM100-159183R. Here are the links:
http://www.managemyhome.com/mmh/lis_pdf/OWNM/L0709012.pdf
For the manual, including parts breakdown, and
http://www.smallenginesuppliers.com/html/engine-specs/tecumseh/Tecumseh_L-Head_Engines_Service_Information.pdf
For service manual.
The carb kit is part #632347.
Peace,
Clarke