Hey guys i am new around here and not really speedrunning this game but i found something that might be useful:

After you shoot Freddys bike during the chase and it catches fire immediately stop, get off your bike and run away from it (about 10m should be enough) before the cutscene starts. As long as neither CJ or OG Loc are to close to the bike it should not despawn after the cutscene and you can use it for the long drive back to Burger Shoot.

I am not 100% sure if it works as i have quite a lot of Mods installed here but they should not affect the bike despwan (i think).

Its only a PCJ but still better than getting a random car in that part of the city and you can use it for Life`s a Beach as well so it may be worth a try.