dont go back to the shop that said it will sieze up with the wrong plug
while a recent thread here did prove plugs do affect operating temps to some degree, there is no way the incorrect plug can sieze an engine other than due to mechanical interference. ie, its hitting the piston... or you put something like a 4 in where a 13 is specified... in which case the plug will melt first. once again, mechanical interference
theres always a few things to do first to these dirty contraptions..
the plug cap will last all of three miles. replace it. either use an eyelet on the end of the wire and (preferably) an ngk bR6hs. note the
R for resistor. and the terminal needs to be a screw on type, not solid...
either that or change the cap for a decent one from a standard motorbike. ngk being top choice again. get a resistor version. 5k max.
plenty of ways to fix that issue. springs, caps, etc.
the plug itself. the heat number, at first, is irrelevant. stay with ngk 5-7. thread size is a B. projected tip has a P, resistor version has an R, plus a few other letters for variant hex's etc then the heat numbers. followed by a H for the 12mm length of thread. an E has 19mm and wont fit. then S for standard, and other letters for electrode patterns and metals...ie, IX is an iridium tip.
so B(P,R)5-7HS(or ix etc).
the number is how hot the plug gets and whether it cleans itself or constantly fouls. cold weather, short rides, low throttle, run a lower number. hot weather, long rides, full throttle, run a higher number...
you mention your tank leaks. this is usually around the studs. an idea to solder them on, with a washer for extra strength. use acid core solder and a hot iron.
the symptoms you describe, bogging down and losing power...check under the magneto/clutch cover for oil. sounds like your crank seals are leaking.
spark is either running or not running. maybe the occasional splutter cough and surge as somethings burning out...
to check for spark, just hold the plug on the cylinder and spin the engine. it will be apparent if its there or not