Helping to build a rack mount for your wife would be fun. Have you thought about what system you will go with? Friction drive is cheap, light, and simple, if hard on the tire. Staton's 18.5 to 1 gear box/chain drive is strong, bullet proof, and very high quality, but fairly heavy. The GEBE belt drive system is light and smooth with a nice choice of 4 final drive ratios, but the price seems a little high for what is included. I've built bikes using all 3 and the GEBE may be the best compromise between weight and ease of use.
I haven't even started yet. Money will be very tight untill after the holidays. I've decided to go with a one size larger Walbro carb of the same series as stock, porting the intake/gasket to match the carb, and building a freeer flow exhaust. I want to keep the reliability, long life, and fiddle free running of the engine. The only difficulty these mods could cause is over revving the little mill. If I add a tach the revs will be my responsibility and I will have to learn to exercise some control over my exuberance.