90 Compression on broke in rebuild

Machining those parts is not reversible, so I would look at everything else first. Were the pistons factory? Determining which to machine, the head or the cylinder, will depend partly on if the piston is flush with the cylinder at top dead center. If it is flush, you won't want to machine the cylinder, as that could raise the piston in the bore enough to impact the head. The machine shop should be able to guide you. Going to a thinner gasket would be the easiest thing to try, and probably cheapest. I like cheap and easy

I had 4 Polaris Edge sleds, every one of them was hard to start when hot. I had to hold the throttle open about half while pulling the rope to get 'em going most times. That kind of became the norm for starting procedure for me. I feel that when the engine is hot, the gas in the carbs vaporizes and flows into the crankcase, causing an overly rich mixture on starting. Opening the throttle seemed to help that a lot on my sleds.

I would go through the carbs and make sure things are 100% there just to make sure. Pilot jets and the air and fuel passages to the pilot jets are really prone to getting gummed up, which can also cause starting problems.

You need to use a specific tool to measure the cylinder. It's called a bore gauge. They usually come as a set and can cost from around 50 bucks to well over $200. The more expensive the set, the better they seem to work, to a point.

Take the measurement with the gauge in line with the connecting rods. Measure at least three places in the bore; top, mid, bottom, and then measure three more times at 90 degrees from the first measurement. That will give you an idea of whether the cylinder is round, tapered, or oval.

Your machine shop should have checked for out of round when they worked on your engine, as it's a pretty common measurement when seeing what is necssary for a rebuild. If you do go over-bore, you will need to send the cylinders off, as most shops won't be able to bore the nikasil and re-plate it .Nikasil is a plating and is very hard. The plating is not very thick, and if it is bored out, it exposes only aluminum underneath, which cannot hold up to the combustion process. Millenium Technologies and US Chrome are two places that have good reputations for doing good work, replating and repair are their specialties. Usually, the cost is about half the cost of a new cylinder.

If your cylinders are out of round, you can also send them off and have them bored back to round and put back to the OEM size by those companies. Send them the pistons/rings and they will set up the piston to the cylinder which saves you a step.