I'm a little late to this thread but man what a great build!!!!! I hope to see a posting soon with the whole engine sitting on a stand.
As to the issue of tap clearance much earlier in the thread I'v had similar issues at work in the maintenance of equipment. First if you are looking fo along taps they are commonly called pulley taps in the USA. As noted they are expensive but worth it if you have a lot of jobs that need a specific tap size if not you have to improvise. Some ways to improvise:
The extension shaft trick is well known but my twist on this is to use Loctite to hold the tap in place. This allows for a reasonable amount of clearance to slide the tap in. Depending upon the Loctite used you can have a working tap in as little as an hour. More importantly no heat related issues, even if you need to apply a bit of heat to replace the tap. Also you don't need to go that big on the shaft size 1/2mm walls can handle the smaller trap just fine.
Now some may barf at this idea but a common wrench on the tap can work if you have a way to hold the tap vertical and steady it. Usually this means a finger on the top of the tap to steady and a really short wrench to maintain feel. With care you can tap difficult to get to items especially ones that don't allow for an extension. You may have to make a wrench though to fit the taps square end.
plastic tubing (air line tubing) can work if you have it in a size that can allow for the tap to be pressed in. This is likely an act of desperation but if you have no other way to get the tap with a driver it can work with small taps. It works better to clean up existing threads and you still need a hand or fingers on the tap to guide the tap square. Basically you have a flexible shaft.
Go down to the local hardware store and buy 1/4" or 3/8" drive extensions for ratchets. Cut off the male square end, drill and face for the tap of interest. The female sock allows you to use common mechanics tools (ratchets, breaker bars, screw driver handles & etc) to run the tap in and out. This can be a quick and dirty way to get an extension without the extra work of fabricating a shaft and a way to drive it. Often these extensions can be found on sale new but more importantly they are dirt cheap used. Thrift stores are the place to look and the cheap ones made of cheese steel are easy to drill. Basically an extension shaft with support for mechanics tools built in.