Really really helpful to post a PHOTO
That said,1; non gasoline dissolving silcone gasket
sealer should last awhile assuming not high vibration area.
3;match material super carefully heat both areas to critical state just short of melting then drip onto cracked area.DifFicult without PRIOR EXPERIMENTATION.
4; Convert to external tank.Often just a simple outboard
pump hose into a jug attached (coupled into) to fuel
input hose will do the trick.If jug ugly enough nobody will steal it.
All the Best