The relevance of heterogeneous architectures has significantly increased over the last decade due to stagnation of performance scaling. Concurrently, increased performance-energy tradeoff requirements driven by the growth of edge computing, with a large focus on Artificial Intelligence (AI) inference, further motivates efforts towards hardware customization. In this context, the open RISC-V Instruction Set Architecture (ISA) and its custom extension oriented paradigm are a relevant technology towards this specialization. However, customizing a processor is a lengthy process requiring Hardware description language (HDL) expertise. Furthermore, for validation and simulation purposes, implementing an Instruction Set Simulator (ISS) of the modified core may also be a necessity. This introduces the need for development of two unrelated codebases, increasing development time and effort. In this paper, we explore High-Level-Synthesis (HLS) to realize both the hardware and the respective simulator through a single codebase, which reduces design effort and simplifies specialization of a RISC-V through specification of custom instructions at high-level. We present a C++ based design of a RISC-V core, and validate it as an ISS, as well as a hardware module synthesized for an AMD Zynq UltraScale+ Field Programmable Gate Array (FPGA) through HLS, which we integrated in a System-on-Chip (SoC), and functionally validated through a state-of-the-art set of unit tests.