Chuck, welcome to BFs and Folding Bike in particular.

More likely than not, your crankset utilizes chainrings which are 130BCD. Most modern road cranks (Shimano, SRAM, FSA, Cannondale, etc.) are 130BCD, with the exception of Campagnolo which are 135BCD. Yes, a compact crankset (50-34 which is on your road bike) uses 110BCD, but since your current set up is 55-44 most likely it is 130BCD. Telling us the brand name of the crankset will help nail this down.

Search ebay for 130BCD chainrings. Anything close will work. For instance I run a non-stock early 1990s Shimano 105 53-42 on my Dahon with a 12-26 9 speed cassette. I ride it primarily along coastal beach areas when I vacation, so I don't need low gearing except for head winds, and this works for me.

Think about which combo of chainring and sprockets you utilize the most. For instance, if you generally use the 55T with the middle set of sprockets, dropping to a 54T or increasing to a 56T won't make a large difference. If you primarily used the largest sprockets with the 55T dropping even to a 53 or 52 might prove helpful in taking advantage of all your gearing.

One piece of advice, if you've not already thought of this, replace your chain and cassette at the same time you replace the chainrings. Often times new pieces in the drivetrain don't play nicely with the old pieces. Also check your jockey and pulley wheels on your rear derailleur. They might need changing as well.

Best of luck on your overhaul.

PS--- Found this picture on the internet of P18, and it appears the crankset is an FSA which looks like a 130BCD to me. By any chance is this a close match?