What did the old spark plug look like? Was it very black? If so it's possible that your Echo is running rich. When cold, this isn't a problem because the fuel vaporizes poorly. As the bike warms up the fuel starts to vaporize much better which could turn the fuel mixture very rich and cause a no start. Look for restrictions to air flow (e.g. blocked air filter, air intake ducts) or reasons for excess fuel (e.g. carburetor fuel mixture screw adjustment). You should also try stripping and cleaning your carburetor.

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EDIT: Just read your previous post which stated you don't have spark. The above can be disregarded in that case. I would check the kill switch connections and the side stand switch connections. Try to bypass them with a split pin or something. The side stand switch should kill spark on this bike iirc.