Replacing extrusion nozzle on legacy reprappro mendel

I recently had a hot end meltdown on my legacy mendel from reprappro and rebuilt it using a .3mm nozzle as the .5mm was out of stock.
Since then I've been unable to get consistently good prints, with the PLA either not sticking to the heated bed or the layers not bonding together very well.
I decided to go back to the .5mm nozzle but obviously can't get one from Reprappro as they have shut shop.

Does anyone have any suggestions on either:

sourcing a replacement .5mm nozzle. (I have the original style one piece hot end, NOT the newer quick set)

replacing the hot end with a standard type (which type to get for the easiest installation)

or what might be causing the .3mm prints to be so problematic?

Thanks for any suggestions.

Lapo

Although i had too many problems with my filament at that time with 0.3 ( quick set in my case ) was a perfect but slow solution for me. The problems you are having have to do with layer height in my opinion . I dont print pla but the print bond problem is worst at abs because of wraping

First thing i would suggest is to look at the layer height in slic3r ( if that is waht you use ) . Then take a good look at ur table . Look again z home . I think you ll work it out.

When i was without a nozzle ( broke one .5 quick set ) i managed to drill .5mm in a bolt to make on. I think ur nozzle is much eaisier. So find a lathe and asked them to do a nozzle like urs.

And what do you mean meltdown . the ptfe tube melt in the nozzle ? There must be a way to clean this up.

Thanks Gaou for the suggestions. I agree with you that the layer height may well be to blame, but it seems to be that the .3mm nozzle is a lot more temperamental to the .5mm. Its also very slow in comparison hence my desire to go back to .5mm.
I am tempted to ream out the nozzle with a .5mm bit but I'm afraid of damaging it now that I can't easily get a replacement from Reprappro...

With regards to the meltdown, the thermistor got disconnected while the printer was unattended and, as I had been running the original firmware, the hot end overheated to the point of splitting and warping the aluminium block. The (2 piece) nozzle also looked to be damaged from the heat but possibly I could try and resurrect it.

My preference however would be to find an alternative source of nozzles or switch to an off the shelf solution.
With the latter, is there a recommended alternative hot-end which can be fitted to the melzi driven Mendel? I'm happy with making physical mods to the printer. Less comfortable with electronic changes.

Thanks.

Lapo