Note, I'm a fourtrak owner, and have not seen a sporty engine, but have mostly worked on petrol engines through my time ....

First check for a few tell-tale signs that your new gasket isn't leaking: white 'froth' on top of your engine oil, it will be visible on dipstick (indicating coolant is getting into the oil), a bubbly scum floating on top of your coolant when viewed through the rad cap (indicating oil is getting into the coolant), or a visible plume of fumes rising from the oil filler cap if the cap is removed when engine is hot and running (indicating compressed gas from combustion is getting into the crankcase through the oil chambers - knackered piston rings will also cause this effect). Any or all of these might indicate the new gasket has not sealed properly - take the head back off, smear some gasket sealant put it all back together again.

Assuming there are no obvious indications that your new gasket is leaking, then I fall back on logic ... obviously you've had the head off and put it back on, and assuming you didn't adjust anything else (did you have to remove the distributor? if so, then check ignition timing), then I can think of only two factors that may have been inadvertantly altered, which of them depends on the sportrak engine configuration of which I am unfamiliar:

If the sporty has push-rods up the side of the engine and rocker arms on top of the head, then check your valve clearance; if the new gasket you put in was 1/10th of a mm thicker, then your valves will now be opening 1/10th of a mm less because the rockers are now that much further away from the camshaft.

If the sporty's block is an over-head cam arrangement, then I can only suggest you check the cam timing, make sure it didn't end up a tooth out when you put it back together.

Hope this helps

Duncan