Before you put much work into it, I would suggest doing a compression check. If I remember correctly, around 75 psi and up is good. 85 is better. Compression under 70, and you will have a hard starting motor, as well as one that won't idle down.
I v'e only found one TN with a good powerhead over the years, and many that are worn out... due to high hours, or running them 50/1.
Jerry Ahrens AOMCI Member MAPS Chapter President