Sounds like you got a new defective carburetor. Happens sometimes. The needle and seat may be bad, or you may have a leaking float. The float should lift when the fuel level rises and push the needle into the seat, working as a valve inside the carburetor to stop the fuel flow. Doesn't appear to be happening in your case.
The hard turning-over is most likely a valve adjustment issue. Since you didn't mention any running problems after it was started, it sounds like the valve is not opening properly to provide a compression release. You can check this by watching the action of the valves with the rocker box cover removed. You should see both valves open and close fully, then one will open slightly as the camshaft compression release functions. If it's not an adjustment issue, you may have a faulty compression release on the camshaft, but odds are the adjustment will solve it. Both intake and exhaust valves are set at .003" on a cold engine at top dead center on the compression stroke.
Hope this helps. Let me know...
I got the carb fixed - I had installed a gasket incorrectly. I still have the compression release problem. I have observed what looks like engine oil blowing up into the base of the air breather through the breather hose from the engine - is this sympmtomatic of the valve problem? Unfortunately, I have to go out of town again tomorrow & won't be able to get back to this for a couple of weeks - how can I get back in touch with you?
The oil is probably due to your carburetor issue. I would smell the oil and look for overfilling. If it smells like gas and/or is overfilled change the oil and get it back to the right level. Clean out the breather system as well as you can as well. Once you do that it may smoke for a while but should not damage the engine.
When you get back, just reply to this question and we'll continue from there.
Have a good trip. Hope it's for pleasure and not for business. I've done a lot of business travel over the years and although it had it pluses, it sure wasn't always much fun. If it is for business...
Best of success!