Good luck with this one, L drive was only made for about 2 years and parts are had to find. The power plant is a Force power head and lower unit but with a custom transom penetration. Have you googled L drive parts, should be able to get some hits? Harder to find item are for this custom transom assembly such as the bellows. Over the last few years there has been folks like you who have bought one of these and asked questions and even got it running but they shortly disappeared.
I would first inspect the bellows and other items that may be bad and see if you can find a parts source before putting any more money into it. Then look for a L drive or Force manual to help troubleshoot.
1997 2400 SCR
5.7 Vortec / Bravo 2