Most guys have had good luck with ASP, but it does have its haters. The brands that rarely cause trouble are OS and Thunder Tiger. Evolution and Magnum seem to have good reputations these days, although older ones didn't so much.
The .60 LA won't make as much power as a ball bearing engine. You probably won't be happy with it.

To fix your MDS, it sounds like you have a fuel supply/pressure problem. Here's a rundown of stuff I would check:

Running muffler pressure? Is the line clean?
Fuel filter clean? (you are using a filter, right?)
Holes in any lines or leak in tank?
Fuel line pinched or kinked at all?
Tank mounted near the engine without excessive fuel line length?
Correct type of fuel line in use?
Clunk moving freely near but not touching the back of the tank?
Clean carb (as already suggested)
HSN tuned 300-500 rpm rich off peak RPM?
LSN tuned so as not to load up and to allow good transition?
Carb o-ring, rear plate and head sealed airtight?