I don’t have a recommendation, but I will be watching this thread with interest. At one time, I was very interested in either having my head professionally-ported or simply purchasing a head already ported and equipped with larger valves, etc. Here are a few of the observations from my own experience; your mileage may vary:
Grom FatherZ: Pretty much the gold-standard of head-porting and hopping-up Grom engines for maximal performance......but reportedly very pricey. Website doesn’t list prices for head-porting services, and no email. Communication very limited. They don’t seem to do email or phone. I wanted to send my head to these folks, but in the end, my questions (sent via PM on forum) went unanswered and I never even got a firm price quote. I think they primarily deal with much bigger wallets than mine.
JC Racing: Decent website. Moderate to high priced head-porting. I couldn’t find any Grom owners who had actually used their services, however. That concerned me. No response to email inquiries.
REN Motowerks: Nice website. They listed a new factory head which had been optimized, larger valves, etc. This looked like a really cool solution, but I think they have largely stopped doing business. Email inquiries went up-answered. Probably out of business.
DHM Racing: I liked this vendor; he re-flashed my stock ECU. Decent website. Like REN, DHM lists a new re-worked and optimized head, but has shown out-of-stock for months now. No reply to email inquiries. I think they may have ceased doing business.
Hard Racing: Extensive web-site, outstanding customer support. Promptly answer emails and — in a pinch — even phone calls. The issue for me here, however, is that Hard Racing just sells the Finbro heads, and those heads have had a number of pretty well-documented issues. While some folks have used Finbro products without problems, I don’t feel comfortable with a Finbro head from what I have seen.
This is about where I left off — essentially I gave up on the idea of having my Grom head ported. I will be curious if others can provide some more workable options.....maybe I’ll reconsider the idea one day.