One thing you can try is to check the valve clearance. It should be .007-.009 on both inlet and exhaust. I just use .008 and if a .009 will not fit after adjustment its correct.Blow-by = leakage between the piston rings and the cylinder walls, valve seats have nothing to do with this sorry. You could have a very small amount leaking past the valve guides but that won't cause hard starting.
How many hours are showing? these engines will usually give 5-6,000 hours of life before they need re-conditioning.
The only real way to check the engine is to pull it out, remove the sump and head, pull the pistons out and check the bores and pistons for wear. Sometimes you are able to get away with giving it a new set of rings and a flex hone of the bores but you need to make sure the bore is within tollerance. If you want to check this out, i have all the measurements at home.
The other thing you can do is a compression test. I had a machine that had low compression and was a little hard to start but when warm it was great. If you want it just rite be prepared to spend 1-2 grand. If you can live with it, live with it, if not they can be repaired. Don't over-look aftermarket parts!!! they are less than 1/2 the price of genuine but i think they still use genuine piston rings.