Use Xilinx’s MicroBlaze ?
NIOS II is Altera(Intel) 32-bit softcore RISC
MicroBlaze is Xilinx(AMD) 32-bit/64-bit softcore RISC.
Both are closed source, and not going to be available to be licensed on their competitors FPGA.
Or you could use an open source Verilog, SystemVerilog or VHDL RISC-V core instead. There was a branch in github to migrate the LimeSDR-USB gateware away from the proprietary NIOS II to a RISC-V core, but has not been updated in 5 years, so I suspect it might be a dead end (anyone know more ?). Or look on opencores for other softcore options.