You need actual charge control logic for a modern rechargeable battery so that it doesn't overheat or malfunction, but you can get multi-amp Li-ion/LiPo chargers online for ~$25 if you're able to find and attach the right connectors.

This also seems like a situation where the maker community could easily open source replicate whatever the custom connector to the bike requires.

You can't just hook up any lipo charger. They need to be able to charge and balance the cells individually, which could be simple or extremely difficult depending on the arrangement and connector.

Charging batteries is usually a bit more complicated than pumping DC into them.

In this case one assumes the battery itself isn't odd, but it uses a different type of cable or similar (Don't know what the capacity is but I assume it would be high enough to need serial and sense lines on the charging cable)

LIPO chargers require logic because they have to switch from constant voltage to constant current. Also, while required, you probably want to monitor temperature as well both because charging when it's too cold will damage the battery and LIPOs have a tendency to thermally runaway when they start to fail.

Read: catch on fire, sometimes leading to a pretty violent explosion if they are packed tightly into something like a bike frame.

Having worked with high amp LiPo cells I have developed a unique appreciation of both their peak current output and their ability to strike fear into the everyone around them if you accidentally short them or start to see them ballooning/bulging.

My only run in with said fire was thankfully fairly minor, I spotted the pack looking a little buldged and summarily ripped it off the charger and dipped it into a 44 gallon drum. After about 60-90s it started smoking and caught on fire.

Don't fuck around with LiPos.