You most likely have a Eureka Flex model with the earlier radial finned cylinder and head.
You need to check some basics and get that thing running a consistant 25MPH + before attemtping any upgrades.
-Make sure the ehaust baffle and header are not restricted
-Make sure your carb and fuel system are clean
-Check the color of the spark plug (tan to a little chocolately is good)
-Check compression with a gauge if possible
Garellis that have any air leak around the head gasket or cylinder base basket have a tendancy to run good for a minute or 2, then heat up excessively and lack power.