Spare parts business for the car companies is very expensive.--Packaging , warehousing, retrieval and shipping of 1 part all costs money --They are more interested in assembling new stuff from bins with a 1000 parts each.
A marketing thing.-----A 15 hp sold as a GT 10 or powerful 10 HP.----Some lakes are limited to 10 hp.-----And the factory decals say 10 HP.----Easy to me.