I don't know if it works on 64bit but you can try to enforce 8087 code using: -CfX87
Delphi mode is worth trying: -Mdelphi
Edit:
I suggest that you use 32bit Lazarus/FPC and to produce a 32bit conversion first.
Yes, I use the 32bit Lazarus, and target is a 32bit exe.
I first added the -CfX87 but the crashes do still occur.
The -Mdelphi is not required because I do a file-by-file decision on this.
Lots of units do already work in the objfpc mode
since I {$IFDEF} the differences betweeen Lazarus and ol' Delphi 5 ... ;-)
The whole project does compile already, since a week now.
So I must track down to the root of the first exception that occurs
to find out what's going wrong. There's lots of Set8087CW() in the code
and maybe I have to replace all these calls with SetExceptionMask()