Welcome aboard. Sorry you're having trouble.
So a couple questions to clear things up. You said it was running fine last week. Was this before or after the top end replacement?
The case leaking oil, I assume you are talking about transmission oil and you replaced the clutch cover gasket to address the leak?
Also, were you able to ascertain the reason the top on blew in the first place? Most common reason a top end let's go is due to an air leak at the crank seals, base gasket, or intake gaskets causing a lean condition. Top ends generally don't pop for now reason. And if that reason was not addressed in the rebuild, the new top end will be destined for premature failure.
Top end replacement, did you use quality parts as opposed to Amazon Chinesium? I only use Wiseco replacement pistons and quality aftermarket gaskets and seals. Cheap parts have their place, but many times going with cheap parts on things like a rebuild will end up costing you in the longer run and sometimes even the short run.
The bogging.... We need to figure out if it is a lean bog or rich bog. Being that you state you are new to 2-strokes I assume you might not be sure how to tell.
A lean bog will be a smooth loss of power and there will be very little to no smoke coming out of the exhaust.
A rich bog will be sort of blubbery, kind of sounds like the engine is drowning. This will also produce excess smoke out of the exhaust.
Does your Blaster still have the TORS system in place? This is a safety system designed to interrupt the ignition in the event the throttle sticks open. If this is malfunctioning it could cause issues like you are describing.
Have a go at these questions and we will start figuring out where your problem lies.
