Get["FeynCalc`"]

Scalar Products

ScalarProduct[p1,p1]=mN^2;
ScalarProduct[p2,p2]=mN^2;
ScalarProduct[q,q]=qs;
ScalarProduct[k,k]=mPi^2;

ScalarProduct[p1,q]=(s-mN^2-qs)/2;
ScalarProduct[p1,k]=(mN^2+mPi^2-u)/2;
ScalarProduct[p1,p2]=(2*mN^2-t)/2;
ScalarProduct[k,q]=(mPi^2+qs-t)/2;
ScalarProduct[p2,q]=(mN^2+qs-u)/2;
ScalarProduct[p2,k]=(s-mN^2-mPi^2)/2;

(*const=gA*2*mN/(Sqrt[2]*fPi);*)
const=gpinn*Sqrt[2];
m=FV[eps,mu]*const*F1p*SpinorUBar[p2,mN].GA[5].((GS[p1]+GS[q]+mN).GA[mu]/(s-mN^2)+((2*FV[k,mu]-FV[q,mu])f1-FV[q,mu]((t-mPi^2)*(1-f1)-sigma)/qs)/(t-mPi^2-sigma)).SpinorU[p1,mN];
m=m//Contract
m=m//FCI;

mDagger=ComplexConjugate[m];
mDagger=mDagger/.sigma->sigmaBar;
mDagger=mDagger/.eps->epsBar;
mm=m*mDagger

mm=1/2*FermionSpinSum[mm];
mm=DiracSimplify[mm]

SigmaL

amp=mm;
amp=amp/.Pair[Momentum[eps],Momentum[epsBar]]->dot[eps1,eps2];
amp=amp/.Pair[Momentum[eps],Momentum[p1]]->dot[eps1,p1];
amp=amp/.Pair[Momentum[eps],Momentum[q]]->dot[eps1,q];
amp=amp/.Pair[Momentum[eps],Momentum[p2]]->dot[eps1,p2];
amp=amp/.Pair[Momentum[eps],Momentum[k]]->dot[eps1,k];

amp=amp/.Pair[Momentum[epsBar],Momentum[p1]]->dot[p1,eps2];
amp=amp/.Pair[Momentum[epsBar],Momentum[q]]->dot[q,eps2];
amp=amp/.Pair[Momentum[epsBar],Momentum[k]]->dot[k,eps2];
amp=amp/.Pair[Momentum[epsBar],Momentum[p2]]->dot[p2,eps2];

amp;

amplitude=FortranForm[amp];
f=OpenWrite["/home/rperry/Dropbox/projects/perry_Fpi/amplitude.dat"]
SetOptions[$Output,PageWidth->10000];
WriteString[f,ToString[ amplitude]]
Close[f]