oh, cuba on an ural. the ural is the most common bike on fidels island.
to rent one will be difficult, as it is forbidden for cubans to rent their private bikes or car to tourists. to buy one is also not practical, because prices are very high. for a good one the seller will ask around 4000 to 5000 US.
shipping your own bike over is an option. on my rtw journey I flew my Yamaha Tenere into Havanna airport. No drama getting the bike out legaly, only 10 usd and two nights sleeping in the airport.
shipping your bike in with a sailboat is also fairly easy with the customs, I know that because I left with a sailboat and friend arrived with their Harley on a sailboat.
I dont know about ports. But legaly you can take your bike into Cuba, the Customs at the airport in Havanna know everything about it.
I dont know how your spanish is, but you should be able to communicate in spanish because this makes it a lot easier.
No, I did not like Cuba and I left after only ten days. But go and see for yourself.